Let's call my player Aggressive Alex. He is a power-gamer. When the game calls for "a low-magic campaign centered around a frontier town of humans and elves," he asks to play an elemental-blooded demihuman fighter/mage who has a split personality and one is helpful and the other is a serial killer." When we play a higher magic game, his character is the half-tiefling, half-giant child of two gods. He declares that, because he is the child of two gods, he's the leader of the party. He lacks the real-life charisma to make such a proclamation and sulks when no one "follows him." As the GM, I should have had a better handle on this. Alas, all I can say is that I've learned a lot over the years.
I'll have to do a bit of backstory to set up my worst moment. For several years, I ran all my campaigns in the same universe, with each campaign being set roughly 50 years after the previous one. Oftentimes, this meant that victorious PCs from one game would show up as deities in the future, which the players loved. One of the storylines dealt with a powerful artifact that could kill gods, but as the focus on that epic level campaign, the party destroyed the artifact. Yay for them. Moving on.
I've only ever had one PvP encounter in a campaign. Aggressive Alex attacked another PC while they were on a ship. The battle ended with the ship getting destroyed and everyone died. Alex quit after that. Since he wasn't playing any more, I ended up using his PC as a villain later on (the party had been resurrected to continue the story). Eventually, the players won the day and their own characters became gods.
Alex eventually came back to the game months later when we were playing something else and (I thought) everything was reconciled.
I was getting pretty burned out at this point having DMed for several years, so I suggested a new game with a rotating GM style; basically, each player would GM for two sessions before turning it over to the next player to run. I hoped this would get everyone interested in GMing so I could have a break.
Things were going well until the last player took his turn as GM. It happened to be Aggressive Alex. Keep in mind that this was a low-level campaign; I think the party was level 4 or 5.
Alex's storyline was thus: the gods are suddenly forced to walk the earth again and the party tries to help them. The villain turns out to be his slain child of the gods PC from the old game who somehow manages to rebuild the godkilling artifact (the one that they spent an entire campaign destroying) and all the gods that were previous PCs are helplessly trapped in front of him. He proceeds to execute each one before engaging in the "final battle" with the level four party. After it was over, he wondered why everyone was displeased with his turn as the GM.
On the one hand, it's not like I (as the actual GM of that setting) can't just easily retcon that story out of existence. On the other hand, it was the single douchiest power trip I've ever seen. Also the most ridiculous.