There are some things I just want to punch in the face.
As part of my university degree I do things in a physics lab, and also in this lab is a guy with... a face. It’s not ugly, as such, it’s just he has these huge lips and a big nose and something about them makes me want to punch him. (Lately he has been growing an afro and a little moustache, which is not helping at all.) I also have an acquaintance who insists on responding to things people say with “you do realise that...” when he isn’t asking a question.
I would also enjoy hitting at least half the population of the TV Tropes forum.
The top of the list, though, is evil PCs.
I will admit I am a little biased. The first ever time I DMed a friend of mine (well, ex- now, she cut off contact with me years ago over a D&D argument of all things) decided she wanted to play a Chaotic Evil character, and since this was a first-level game she ended up nearly killing another PC so she could loot a masterwork scimitar. (On the upside, she swiftly got bored and rolled some kind of snarky halfling instead.) Since then I have not seen evil PCs played much better.
The main problem, I think, is that nearly all of them were played with the same goal in mind: create party conflict, screw over the other PCs if they can get away with it, and generally shit on the game. Contrary to what some people might think this does not amuse me.
There isn’t actually much that I mind, as far as games getting screwed up go. My players have told me they feel they aren’t on rails and can do what they like, and half of my adventures start with me sitting down and saying “Okay, what kind of adventure do you guys want to have?” and rolling from there. (Generally they take turns picking adventures related to their character’s interests.) If they happen to find something fun then sure, whatever.
What bugs me, I suppose, is when one person’s idea of fun is something that happens at the expense of others.
You probably know what I mean. The party rogue killing the party when they are low on health just before the campaign’s end is a pretty unsatisfying ending for everyone else, finding your wand of healing relocated to the party kender’s pocket when you really needed it is not enjoyable, etc. (Then again, it is probably your fault for partying with a kender.) It’s especially annoying if the person in question brags (“oh man, I screwed everyone over sooo good last week!”) or cries (“You didn’t like that? What are you, a faggot?”) as well.
Evil characters can work, I’ll grant you that, but you need to play the character well, not just for the sake of starting that kind of trouble, and be prepared to accept any consequences of your actions. And you probably need the right kind of group to work – a group which, should you try to screw them over, won’t feel like they were abused for someone else’s enjoyment.
You probably need a DM who isn’t biased, also.