Oh man, I got one to top ALL of these:
Assholes in videogames. As in, real life people.
I just got out of a game of LoL, with 9 others playing. My team was full of all polite people. Teemo didn't say much, but everyone else was friendly and talkative. And THEN there was the enemy team...
The enemy in my lane was a Kennen who was very skilled. He didn't say anything until the very end, where he was friendly, and was the ONLY person on the enemy team who I respected in any way. Jarvan stole top from Mord, who had called it first. Jarvan was the first to give up and just stand there after we started kicking their ass. Mord was an asshole cause even though Jarvan stole top, Jarvan still chose before him. Mord could have gone a support champ, which is what they needed, but instead he went support Mord. Which doesn't work well, by the way. Ezreal was whiny and incredibly unskilled, but acted as if he was the best thing since sliced bread. When I was friendly towards the enemy team like "Hey guys, come up, it's a game, have fun!

So only one player in that entire team was a good person. Imagine YOU being in Kennen's position, the only good person, amongst bickering resentful trolls. Obviously, it's rather annoying.