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.