Alright, so recently my group has started to develop SEVERE problems with one another, almost all stemming from a single player at the table. Now, I'm not th DM for the group, but I'm the closest thing to the "Head" of the group as I'm the one who brought everyone together, and works out the scheduling and everything else, so I feel like it's my obligation to be the one who deals with the problematic player before things get way too out of hand.
We've been doing fine for the last two years, but in the last three months or so, the player-who-shall-not-be-named has been throwing fits at the table, throwing dice when he's on a losing streak, and almost every session he attempts to pack up his things and straight up leave in the middle of the game because things aren't going the best for HIM. He's also been bringing problems with players outside of the game, into the game. (i.e He called shotgun in the car, someone calld blitz, and he has to sit in the backseat, so he reached for the absolute longest stretch he could in game to attempt to kill their character.) and after enough attempts on the other PCs lives, the characters took justice into their own hands and killed his character, which just so happened to be his favorite.
Now don't get me wrong. We all love this guy to death, and so far we're not yet to the point where we're going to kick him from the group, but I recently got wind of the fact that his next character is a character that he's specifically designing to kill off the other PCs in the group because he's angry at us for killing his other character. I know this is going to be a hard turning point in the group, and it's going to cause some serious drama that I'm worried is going to escalate pretty far.I've tried talking to him about the ridiculousness of this, see if there's anything that's been going on that is influencing why he's acting this way, but so far he's stonewalling me pretty hard.
What would you guys do in my situation?
Any advice you could throw my way about dealing with a highly problematic player in an almost set-in-stone group?
"Hey quit being a jackass. It bothers us"
If he does not,
"Hey quit coming to future sessions, we don't want to hang with you anymore."
If he does not,
"911, this dude is stalking us"
Add a bunch of conversations with the dude between each step about how he should stop whatever he is doing.
No group where people gather on a voluntary basis to have fun and create exciting, memorable stories is set in stone. Talk to the player directly and politely outside the context of the game and ask for his help in resolving the issue. If a compromise cannot be reached, part ways.
You mention the player was fine before 3 months ago when all this began to start up. You also mention that this just isn't in the game, but almost at any situation. Something is setting off my alarm bells.
It is very possible there is some psychological or medical condition that is manifesting itself.* I am no doctor, nor do I know the guy, but if his behavior has truly radically changed over the last 3 months, he may need to seek professional help. Also, it may be helpful to sit down and talk with him and see if there aren't some major bad life happenings occuring for him. He may become defensive if asked about this stuff, but if he is truly changing to the worse, as firends, you guys might need to try to help him or get him help.
Now, if he was always a semi-jerk and it has been slowly getting worse (that is, no personality shift or medical/life issues), have a talk with him where you lay down the law, and if he continues to have tantrums, he will be excused from the game.
*in 2011-2012 I started to abruptly manifest volital moods, having issues with comprehension and even bouts of uncontrolled emotions like crying. As a gamer, I sometimes acted childish. On these forums it manifested itself as some truly bizarre posts of a confrontational nature even if I agreed with the poster. Turns out I was(am) suffereing from a form of Frontal Temporal Dementia. Were it not for my friends, some of who I gamed with, I would have ended up on the street. Instead, they made me get help (I was convinced I was fine) and the doctors discovered the issues via MRI and testing. Where the FTD came from wasn't evident for a few years, then earlier this year, the reason for it came clear--> I was diagnosed with ALS.
Hopefully, his issues aren't serious like mine, and most likely they aren't. I simply post this as a cautionary note not to rush to lable this guy as a jerk and kick him to the curb, especially in the light of the OP's post that seems to suggest this change came "all of a sudden."
This doesn't sound like a gaming group issue, this sounds like a friend-having-problems issue. You are going to have to deal with it in that context. In terms of its impact on your gaming, you will decide as a group, consciously or unconsciously, to what extent you are prepared to ride it out until he manages to deal with whatever is really bothering him.
I would also point out that this game can be really cathartic. It may be that he needs something from it more than anybody else right now. Figuring out what that is can be a challenge (he probably doesn't know himself), but if you hit on something he really enjoys, I would advise you and the other players to run with it.
For example, I have a player who has kind of had the crap kicked out of him by life, and really needs to be the toughest guy in the room when he plays, and to not take crap from anyone. As a DM, when I recognize he is going through one of those periods, I run the game in a way that makes sure he gets what he needs.
Don't play D&D with the guy. Do be there to help him through whatever's going on, if that's the reason. These are not mutually exclusive. You're under no obligation to turn "fun D&D night" into "Bob's therapy session" but if the guy's a friend, of course you support him.
That is if he's not suddenly started being a dick just for fun, in which case don't play D&D with him period.
Incidentally: not to be all "it's actually your fault" here, but solving out-of-game issues through in-game actions is universally a terrible idea, and by ganging up on his character and killing him you didn't do any better than he's doing. If nothing else, sit down with him outside of the game and at least stop that spiral before it completely ruins your game. "But no group would travel with someone who keeps trying to attack them" - it's not about that. Even apologize for killing his character and offer to retcon it if that's what it takes - be the bigger person(s) about it and, who knows, that might even be the thing that makes him realize how annoying he's being.
Sounds like this guy has some personal problems in his life. Have a good chat with him.
NONOTBLACKLEAF wrote:
"I'm killing Joe the Wizard because I had to sit in the back seat on the way" is not an in-game problem.
It wouldn't even be a good justification if the game in question had both PvP and cars.
Right, but it was the player in question who did that, not the rest of the players. Read what the OP wrote, then read what you wrote.
Okay so "I'm killing Bob the Fighter because he's trying to kill me because he's being a sourpuss" isn't an in-game issue. There's no separation happening here, it's all players being mad at each other and taking it out on each other's characters, from start to finish. If you gang up on Bob the Fighter in this situation, a) you're willfully ignoring that the character is only doing it because the player's angry at you personally, and b) all it will achieve is that the player gets even angrier at you and makes plans to retaliate further. Which, having indeed read the OP, is exactly what's happening, what a surprise!
Like, okay, it happened in the heat of the moment. That's why you sit down with the guy and say "hey man, that shouldn't have happened last session." Give him an opportunity to say the same by doing it first, letting him save face.
Erawks,
If your friend is so frustrated that he storms out in the middle of a game due to bad dice rolls ... let him leave. Maybe a long walk and some fresh air will do him good. Give him a while to cool off, then check on him. Don't feel like you have to wear a target for somebody else's bad temper.
As noted above, handle this away-from-table. Treat what happened as symptoms of a problem, not the problem itself.
But be aware that the final answer may be that he has to part ways from the rest of the group - hopefully for a limited time but maybe permanently. 
In my sessions people never really get frustrated with bad rolls, because of how I narrate the outcome. I narrate it in such a way, that it doesn't feel like a sucky outcome, but as an exciting outcome. I don't want them to feel like their characters are incompetent. Instead, I want them to feel like their opponents are menacing. In the case that the OP describes, the problem seems to be mostly the player. But maybe a change in narration can also change how frustrated this player gets with a couple of bad rolls.
For example, if you attack a dragon and roll a 1, you could say that they drop their weapon, or the weapon breaks... -which sucks.
Or
You could narrate how their sword clashes into the thick scales of the beast, and sparks fly off the side in a spectacular display. The sword bounces off to the side, and one scale is knocked off, revealing a vulnerable spot. It doesn't seem like the dragon took any injury from the blow though, since the sword was unable to hit a vulnerable spot. And the blow may have dulled the sword, so that it will need sharpening.
The outcome is almost the same, but the latter sounds way more exciting and fun. Another player may follow up on this, and focus on the spot that lost a scale. The DM can then narrate that they get advantage on their attack, if the first player points out the weakness to his party members.