Erik Schmidt's answer is probably the better one to go with (as it'll help you find the root cause), but I'll contribute a bit based on what I see from your description.
From your description, you have a player who enjoys:
And who doesn't enjoy:
And yet, this player is playing in a campaign that is low combat, high intrigue, and where the penalties for building a new character are incredibly prohibitive.
When players are marginalized, they tend to try to get into the spotlight in any way that they can. And that leads to disruptive behavior.
This leaves you with a few choices:
Try to get the player into a campaign that they'd enjoy (either by running an additional campaign, or by encouraging them to find/create a new group).
Remove the player from your campaign.
Find ways to compromise, attempting to bring your campaign and the problem player closer together.
The important thing to realize is that there is no way to force the player to like playing the way you and your other friends are playing.
Character Jumping
Some people like to play the same character, and enjoy the process of growing and developing that character. Other players prefer the creation process, authoring new characters and mechanical combinations.
Dealing with the first sort of player is pretty easy. That's traditional, long-form writing. We see examples of it all the time in many different forms of fiction.
The second sort of player, the one you have now, is a bit harder to deal with. Here are a few things I've picked up to help deal with that:
Avoid introductions
Either introduce the character during downtime ("And so the party rested for six months, and met a new companion"), or use the "poof" method ("of course I've always been a catfolk fighter. Human wizard? Ridiculous."). You want to avoid having players stuck constantly in the untrustworthy rookie slot.
Maintain player parity
New characters should be (or quickly become) the equals of established characters. It's no fun being the one guy who doesn't have an artifact.
The rule of thumb is that anything that's relevant frequently, whether it's powerful mechanically or narratively. A new character can probably be allowed to do without a one-time favor from a noble. But they should probably get written into a life-debt from the king of the current kingdom (or be given something else).
If the new character doesn't start with this stuff, then the stated goal should be to catch them up quickly (within a few sessions).
A level penalty is right out, unless it's a temporary one (or you're playing a game where levels aren't super important). For example, in my 7th Sea campaign, new characters start slightly below the lowest player. But all players are brought to the same experience total periodically.
Let them know that they won't get some of the depth other characters do, but make an effort anyway
From a DM's standpoint, the hardest thing about a character jumper is that you can't be sure of who they will be in the future. So if it takes you three months to author a storyline featuring that character, and they change characters every two, that creates an obvious conflict.
Let the player know about this, and give them an opportunity to work with you. And make an effort to work their current character into the plot from time to time anyway, as a show of good faith.
Combat and Intrigue
It sounds like this player enjoys combat quite a bit. Most likely, it's a chance to see how the mechanics of their character work, and to show off a little bit. This is all fine, except when it stomps all over the intrigue that other players want.
Give them combat
The easy answer here, is to make sure you're providing combat. Don't make talking the ultimate super-power. Give him a mixture of fodder that he can easily crush, and challenging mechanical opponents.
This is where the bit about parity and artifacts above becomes important. If their well-honed killing machine is constantly shown up by the senior characters with artifacts, the combat probably won't scratch their itch for them.
Signal shifts to intrigue mode
Provide clear signals to the group when a shift to intrigue mode is happening. Set intrigue in populated areas (cities, etc.) and combat outside of it. Have combat-based foes rush the group, snarling (or ambush them with a poisoned blade), while intrigue-based foes sit calmly on their thrones.
If nothing else, the occasional reminder that "Baron VonEvil is pretty well connected. Are you sure you want to fireball his face?" can help quite a bit.
Always have a backup plan
This player is going to blow stuff up. As the person who's planning things, that should be a possibility that you consider. How do you keep the plot moving if the villain doesn't get a chance to monologue? What happens if the whole room is fireballed?
The goal isn't to always have a way for the PCs to succeed per se, but simply for the story to always have a way forward. Perhaps that means that a scheme of the late baron crops up shortly after the heroes fail to find the documents, and now they have a fresh chance to investigate.
Or perhaps the important documents were partially sheltered by the lockbox they were in. They've been badly charred, but there's enough there to make out a new location to investigate. Is it a trap? Of course it's a trap. They're going in blind.
Communication
Talk to your players. Not just about how you want them to behave, but about what you're trying to do, and how you're trying to shape the campaign to the individual players. Make it a dialogue, and solicit their feedback.
I'm trying to add in some more interesting "boss fights." What did you think of that last one?
I really liked the way you guys managed to outsmart Baron VonEvil. That's given me some interesting options for the next couple of adventures.
My goal is to tell an epic story of your rise to power. I think we could use a bit more of the political stuff, I just need to work out how to write that.
Etc.
Recommended Reading
There are two excellent sections in roleplaying books that I'd highly recommend for dealing with this problem, if you can get your hands on them.
The first is for the entire group. It's located in the 7th Sea Players' Guide, on page 238 under the heading Resisting the Story.
It's a couple pages long, so I won't reproduce the entire thing here. But the representative part of it is this:
Don't poison the PC pool. Don't act destructively just because "it's in your character," and don't force other players either to ignore your action or kill your character because it's in their characters. Know where the line is and make sure you -- and your Hero -- never cross it.
This doesn't mean that there shouldn't be any conflict within your party. On the contrary, some of the best and most rewarding roleplaying experiences come from moral or ethical disputes between Heroes. As long as you know where the limits are, and when engaging conflict becomes irreconcilable conflict, feel free to pursue arguments with your fellow Heroes.
The Rich Burlew essay Making the Tough Decisions is another valuable discussion of this principle.
The second is for the DM. It's located in the 4th Edition Dungeons & Dragons Dungeon Masters' Guide (pages 8 through 10). It's a breakdown of the different kinds of players, why they come to the table, and how a DM can target each one.
Summary
This is a lot of text, and only really brushes on a lot of important topics. The core assumption here is that you would rather play with this player, than necessarily play your campaign as it exists now.
The key is to give the problem player things that they enjoy. With good communication, a willingness to tune your campaign to the players, and a bit of compromise, the problem player may become a lot more tractable when their needs are met.
It's pretty reasonable you're annoyed. One of your fellow players secretly plotted to kill your character for revenge (and it worked), the DM - the one guy you pretty much have to be able to trust - was in on it, and your fellow players offered you no emotional support at a point when you clearly needed it and instead made things worse for you.
People have been annoyed over character deaths, but you also have the issue of a betrayal of trust.
Different groups (and different DMs) handle stuff like this in very different ways - some handle it well, some badly. You had a conflict within the party, and a conflict between two people, and some character death and secret plots on your hands. These are often things groups don't talk about beforehand, but should. As the DM, I wouldn't have allowed this secret plot and would've talked to you two outside the game to get this enmity settled at first signs.
(And honestly, if this was real out-of-game racism you were experiencing, I wouldn't have tolerated that either.)
So now the guy who betrayed you is the DM and you want revenge.
Don't do it.
A good plan is to decline the invitation, not play, leave, and find something else to do or another D&D group to play with. This is not advice for how to ruin Bob's day. This is advice for how to avoid having your own next two months ruined, and possibly several weeks or months after that as well, and instead have some degree of peace for yourself.
You're pissed off. But, proverbially:
Holding onto anger is like drinking poison and waiting for the other person to die.
— (Not sure who first said this)
So you're going to have several gaming sessions and two months in which you're busy being pissed off at Bob, stressed out working out how to get revenge, and suffering over the lot of it - and expecting him to suffer for it. Eventually. At some point. Maybe.
That's not working out in your benefit.
Here's how the next two months are going to pan out according to this plan.
- Bob's the DM. The universe of the game you're considering bends to his whims.
- He doesn't like you to begin with, resents you for something you did to him, and apparently is subtly vengeful. He might give you a hard time for the next two months. This is going to feed right back into you being even unhappier in general, and unhappy to be in that game. This may even increase the degree of revenge you want, something you might not even get to begin with.
- You're going to be preoccupied being annoyed at him and not really actually just enjoying a good game of D&D. (Not that this game is going to be a good game of D&D for you necessarily anyway.)
- Eventually, you might actually find a way to get back at him. But, since he's the absolute controller of the game's universe, he can just say: "Oh. Well, my guy's goddess smiles upon him and heals him. Then she teleports your character into the plane of fire. Alright, whose turn next?"
- Or you don't get anything out of it and you're just annoyed.
- Or a month in, before anything even really happens, he or your fellow players ask you to leave because you're not being very fun to play with. You probably won't be. You're here for revenge, not to enjoy a good game.
- And you're playing with this guy as your DM the whole time, again. That's worth repeating. Why would you want that?
This entire plan is toxic to you and you alone. It's going to be really unpleasant for you, and more likely than not won't get anything out of it - and if you do, it probably won't be very satisfying.
Don't do it.
But I really want revenge!
This is an issue between you and another person, not an issue D&D will help with. Don't try to solve it through D&D, and don't try to solve it by playing with this guy as a DM for a few months.
Deal with it out of the game somehow. Maybe talk to him and get stuff off your chest. Consider walking away and leaving it behind you rather than let this weigh on you. Different things work for different people. I don't know what will work for you, personally. But spend these two months of your life doing something else. Find peace, somehow.
A surprising option is forgiving him - and not for his benefit, but for yours. Forgiveness is just as often so you can stop holding onto the negative emotions you have - the ones which are affecting you much more than anyone else - and let them go and find some inner peace. You might not be prepared to do that, but I advise you try it.
D&D is normally not like this.
(But sometimes it can be.)
There are going to be groups more supportive of first time players. There are going to be groups where you can genuinely be friends with most people, get along with all of them, and reliably trust the DM and your fellow players. This wasn't one of them, clearly. I suggest you find a group that suits you if you want to keep playing D&D.
One thing that normally goes unrecognised is that several D&D players at the same table, playing the same session at the same time, are usually not even playing the same game. They have different expectations of what a good game constitutes, different ideas of what's OK and what's not, different understandings of the rules, and different opinions over how issues like loot and character death should be handled. Usually, they assume that everyone else shares a similar view, without realising that everyone probably thinks very differently - to each other, as well.
An analogy is having several people sitting around a table to play a game of cards, but they are playing different games: one person is playing Poker, another Hearts, and another Go Fish. That wouldn't work very well, but somehow that's how D&D gets played without anyone realising it.
This disconnect is the reason why someone authored the Same Page Tool - which, as its name suggests, exists to get people on the same page. Its author also wrote about The Roots of the Big Problems and A Way Out (from which I drew the card game analogy). All three of these links discuss this issue and the situations that arise from it, and how to deal with them.
This group was not on the same page as you, and were not what you were after. Another group might be.
Best Answer
Your problem here isn't primarily the selfish individual, but the GM who is letting his out-of-game best friend run roughshod over the game. You need to talk to the GM, first, about how the game is starting to not be fun, then (likely) talk to the GM and the problem player about why.
Be prepared, though; this is probably a lead-up to quitting the group and finding another group to play with. You should assess whether it's worth the conflict to try to stay with this group, or if it's easier to just find another group.