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.
Have dinner with your friend and help her draw up a checklist.
This advise is taken from my own personal experience doing exactly this. And it did help the person I was helping significantly, though as part if it, we also created a character to suit their requirements.
Checklists are an amazing technology that are astonishingly underrated by many people. Functionally, the checklist's role is to enable pre-cognition outside the heat of the moment. With any sort of complexity problems, a checklist is a good way to reduce the granularity of the problem such that a player can concentrate on the novel elements.
So, while I made a checklist for my epic character, with a list of the actions I would take if this or that were true, or on the first turn, the point of the checklist is not to strictly adhere to the thing, but to reduce the need to think about all of the things on the list.
When having this conversation, first discuss common "patterns" in combat. Figure out common situations that she remembers from game. While there are plenty of theoretical situations, it's more important when doing this to let the cognition and inspiration of tactics come from the individual. Just like with making a cheat sheet, the act of making the thing is more important than the thing itself.
After listing all of the common situations that she can remember, work through sample outcomes with her in whatever way suits you two best. (My preference is for statistical analysis, but then I'm very special. One of the simpler ways is to play it through a few times, alternating sides and talking through your decision.)
After each finishes, let her work out a checklist of "if this situation is happening try for X,Y,Z" Then, after a set of checklists are done, let her employ them in game. After she does, make sure to debrief her after game (written or oral, since there is some literature that suggests a written debrief has great learning potential) and talk through what went right, what went wrong, and what needs to change in the lists.
Best Answer
Firstly, you should talk to all your players about the issue outside of a session. You can get some personal impressions first with one-on-one conversations, but ultimately the entire group should sit down to discuss the problems. Make sure the discussion is democratic in nature, though. JohnP points this out in a related question: "The group setting can be dangerous, as it can turn into people digging up old grievances or ganging up on a particular person."
During the discussion you should ensure that everyone is on the same page about how they want the game to be played. The same page tool is a useful set of questions that will drive the discussion in the right direction. Just make sure everyone is fully involved in the discussion and are voicing their opinions clearly (no passive-aggressive bs). If your players are open to compromises a consensus should be reached.
Secondly, make sure your friend is not suffering from My Guy syndrome. If the description matches (and it sounds like it does), show her the link privately and let her think about it. It should help her be more aware of her role as a player and hopefully remedy some of the issues.
You also mentioned your player gets upset as a result of your decisions. This related question contains a lot of suggestions for how to handle players that take things personally, ranging from studying your own approach to kicking out the problematic player.
In the end, though, you are the GM. You are the writer of half of the story, the referee on all mechanics, and the leader in the quest for fun. The way you drive your campaign is part of your style and your players should respect it. They need to be aware that your goal is always to increase the enjoyment they collectively get out of your game. A certain level of trust and respect is absolutely necessary. If this is impossible to obtain in your group, then the group as it is cannot function. Kicking out players or stepping down as GM would be the next steps to try.
Because the GM is so special, though, new players often fail to grasp just how complex the GM role can be, and can at times see him as an enemy and spoilsport. A neat "trick" you can use to show your players what being a GM is really about (that also gives you some rest from the responsibility of being a GM) is to have another person in the group be the GM for a few sessions. It doesn't have to (and most often shouldn't) be the same campaign you're running. Instead, it can be a few sessions of an off-shoot campaign. It's fun for the players because they get the chance to try out new (and often times silly) character builds, and the new GM will discover what it feels to have all this responsibility. Once everyone has GM-ed a couple of sessions, you will all have an idea of who's best at it and will work towards keeping that person as GM in the future. There's a chance it might not be you, but in the end it should result in a better experience overall.
Keep in mind that not everyone is fit for GM-ing, or willing to try at all. Don't force players to GM, and if they decide to try, encourage them to design very short adventures (no more than three sessions). They can always expand on them later if they like it, or end them early if they don't. The player that questions your decisions often probably thinks they can do a better job, so they're likely to accept your offer to prove themselves.