I tried very hard to work with that guy. I knew it was going to be tough when he would hold the party up while he investigated a pendulum trap to try and disarm it when all it needed was a pretty easy dex check to bypass it. He ended up getting killed by it and the party just left him there. Then they were in a town bringing the party cleric to the temple for a greater restoration due to some stat drain. I had the priests collect the cleric and take them to the uppermost floor. That guy followed them up despite the priests telling the party to go find something to do while they fixed the cleric. The priests told him all the way up to not anger the gods and to wait with the party. The priests at the top said turn back now or the temple itself will act. That guy walks past the priests on the stair case, i have the steps dissappear and he falls 80ft, takes max damage and dies, then complains that i gave him no warning and no chance to survive because he was out of ki. The party has my back on this. They were yelling at him the whole time to just it go. I set up a fun session where the parry is being hunted for sport. I let half the party play the hunters as well as thier own characters. They got very into it including the paladin fireballing his own character. That guy complains when his character took damage during the chase amd said he felt he was being targetted.

The last straw was getting him to DM Icewind Dale for us. He was trying to emulate my style of dming where the fights are far and few between but are incredibly dangerous. The difference is that i shower the party with magic items and give them some cannon fodder/flavourful and helpful npcs they can control to help them out. He just hits the level 1 party with a cr 4 monster that kills half the party in one hit and only went down due to two lucky crits. Then he has every single npc act like a complete dick for no reason even though we just saves the town from an incredibly dangerous creature. His last day with us he made fun of one of the other players for his dead wife.

We had a guy in our group who would find any and every reason to bail or show up late or leave a session early. When he would show up, he would just goes rogue (usually playing a rogue) and do his best to ruin the game for everyone else. There was a campaign a while back where I was playing a changeling and he/his PC knew. We were sneaking through a dungeon, my character changed into a goblin or whatever the enemies were to do some recon. He knew what clothes I was wearing and we had agreed on a signal. Also, most dungeon goblins aren’t wearing cool sparkly robes. He proceeded to sneak and kill my character saying “There was no way of knowing which was which”. It brought the whole good down. The DM said I could just bring the same character back, bla bla bla, but it just soured the game for me. I never understood why he acted like that, because it never seemed like he was having fun and it’s not like the rest of us were.

The other “that guy” that I know has gotten better, but he had a really bad habit of taking advantage of homebrew material and hiding or fudging stats/rules. He’d always argue that he could do this or that and would fight with the DM over how much damage he could do. It was just weird, because I never got the mindset of cheating in dnd. We’re all supposed to be playing the same game, chill out my dude.