My suggestions, coming from the other side of the fence where I (and some of the other players) feel that the DM plays a little too fast and loose with the rules, and makes changes to things that we think ought to be "canon" for the well-known world we are playing in:
1) Be willing to consider that the player may be right. Allow him to make a brief argument referencing the rules. Then make a ruling. Make a mental note of how often you rule against the player versus how often you change your mind and agree with him, and try (later, outside the session) to assess whether you're being particularly harsh and/or truly weakening one character's abilities relative to the others'.
2) Be firm if you still disagree with him. If he still disagrees with your ruling, tell him, "I need to ask you to go with the DM ruling for the moment and we can discuss it more later outside of game time, to figure out how we'll play this type of situation in the future."
2a) Try to offer the player another way to reach his objective. Say something like, "Look, the rules say that you give away your position if you attack from hiding. If you then, in full view of the enemy, duck behind the same tree, they are going to know where you are, even if you are so well hidden that they can't perceive you. Thus you do not get the advantages of being hidden in that case. Now if on your next turn you stealthily move to the next tree and hide there without being noticed, and then attack, that would be unexpected and give advantage."
3) Ask players not to use the Monster Manual at the table, and to avoid using metagame knowledge about monsters. That said, try not to mess with well-known monsters in a canonical setting without a really good story justification. If you're playing in a canonical setting, Mummies are going to be something that most adventurers will know the legends of, and the way that Mummies are described in this universe really does preclude a "good-aligned Mummy". If there's going to be a good-aligned Mummy, there should be a good story to go with that, to say how that happened contrary to the usual Mummy creation process, that the PCs have at least been given hints about. Otherwise, yeah, it's pretty appropriate for a PC to automatically kill any Mummy he comes across on sight. They will know the stories....
Note that the 5e MM does say (page 7 if need a reference for your rules lawyer):
The alignment specified in a monster's stat block is the default. Feel free to depart from it and change a monster's alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there's nothing stopping you".
However, unless there is a good story behind the anomalous alignment, and your PCs have access to clues about that story, I think it would usually be better (and annoy your players less) if you either make up a new monster that isn't in the MM, or be clear that you are playing in a non-canonical setting and using monsters that don't match the descriptions in the MM. Even in a canonical setting, you can play variations on less-legendary monsters, but be clear (out of character) with your players that this is what you are doing. In all cases, allow the players relevant checks to recall some in-game, in-setting lore about the monster you are actually playing.
For example instead of just putting in a good-aligned Mummy you could say, "You see a medium-sized humanoid, wrapped in bandages. Make a religion check". Tell everyone with a low score that they think it's a Mummy. Tell whoever got the highest check, "Because of [some detail that they can perceive] you think this might not be a true Mummy but rather a Pseudo-Mummy. Pseudo-Mummies are created by a different process than True Mummies and in some cases can maintain their pre-death alignment." If you want, you can go into the process more, or you can just say that the character doesn't know any more than that. Now you have a good-aligned Mummy that your player shouldn't complain about.
4) Consider having a talk with the players about what game everyone wants to play. You have a conflict in play style with the "rules lawyer" player. Do the others also want to play "his" game, or do they prefer your approach? Can whoever is in the minority live with adjusting their expectations to what the group as a whole prefers? Can there be some compromise?
You have two solid choices
Choice 1: have a talk with the DM (and likely walk away)
No gaming is better than bad gaming(generally). Red flags are:
- He's related to the DM, the DM said he will not do anything about it
as long as his rolls are successful
- decided that his character was going insane, and made himself an
insanity die roller (randomly by the way, nothing happened in game
to justify this happening) {and the DM puts up with that}
- he has the Lucky feat, and gets inspiration for complimenting DM
- You were flat out lied to about the alignment guidance for player
characters.
Address the expectations mismatch with the DM
Before you walk, have a talk with the DM in private. I can't stress enough how important it is that you have this conversation in private. You have an expectations mismatch with this DM, and this table, based on the false pretenses offered to you about the kind of game you'd be playing. You need to spell out the four above points as significant fun degraders for your participation in the game. Find out if the DM cares about your concerns.
If the DM doesn't care, and offers you a "take it or leave it" then
leave. You and this table aren't a good fit.
If the DM listens and offers to make some changes, then stick around
for a few sessions and see what happens.
Addendum to Choice 1: a discussion in private with the grief player would be in order, to explain to them in private how their play is making the game un-fun. Some people do stuff like that and are not self-aware at all - they are nearly oblivious to what it does to the mood of a table since they are too busy enjoying their brand of fun. The point of emphasis for this conversation is that in TTRPG's, the idea is that we all have fun while playing.
Choice 2: make good on your threat in-character (high risk, high reward)
Play on and when the opportunity to make good on your threat to confront or kill the evil character arises, do it. You have nothing to lose. (You can make the last blow a "knockout" blow). Using the knockout feature allows you to make a point without necessarily killing off the other character.
Knocking a Creature Out (Basic Rules, p. 76)
Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.
The worry about alienating people reflects well on you, and there may be more to this social dynamic than you have shared with us, but you are already being put on the defensive.
While I'd usually not advocate a PvP response like this, you have the support of two other players in terms of this isn't going well.
- One of the features of TTRPG is that actions have consequences. You have already in-character made the threat.
- It does not appear that the DM pulled you aside and advised you not to.
This character and this player get to find out that their character's actions have consequences.
When the dust settles, you will have a few more answers to the "Choice 1" options by following through on the PvP threat that you made in-character, and seeing how the DM and the other players respond to that.
Discuss this with the DM ahead of time (in private): since the DM was there when you made the threat, and you don't mention being cautioned about it, put this card on the table. Warn the DM that you will, in-character, follow through on this threat from one character to another. The DM's response to this conversation will inform you as to whether you should go ahead with this, or if you should go back to choice 1 and just walk.
Third Choice
Start your own game, as DM. Invite the others to play. See who shows up.
Out of the Box Choice (which may not fit your play style)
Embrace the evil. Work with the DM to affect an alignment change for your character and go evil. Only you can decide if embracing the dark side is fun for you. Since you note that two other players are tired of this stuff, this is the least likely course of action to resolve your problem.
Best Answer
This player seems to not particularly care about the moral side of his character. But don't punish him for it. Even if he plays the character inconsistently, you should treat that as a creative challenge to you as a DM. You need to be the "straight guy" for their acting up.
Roll with the characters actions as is, do not prevent them either in or out of character, and have consequences play out in game, with little or no "punishment". The story consequences for a Deva that abandons their god and is beholden to an Evil artifact should be quite interesting. Your job as DM is to make them fun for that player and the rest of the group.
For example, I would suggest some "fluff" changes to the character - have their appearance change subtly due to the artifact (make it clear this is the cause, and not something inherent to the character - the character at all times belongs to the player) - e.g their Deva markings glow with odd dark energy, and their voice seems harsh and commanding to those that they talk to. Have Good characters with spiritual connections immediately distrust and/or avoid the character. Sometimes subtle story hints like this will bring a player who wants their character to be a shining example of goodness back on track. However, it should also be OK if the player is just playing for +X stats. They should just see the results of the choices by the way the world responds.
Other than that, you may want to consider that this player is looking for a slightly different style of game (more numbers-focussed) than the rest of the group is looking for. Try to be inclusive, but do check that everyone is happy, and that you are not storing up tensions.