Just general aggression and being a strawman.
The latter I can deal with better than the former, because at least someone can remain calm whilst representing their stance in a grossly inept manner.
I just dislike it when people are constantly angry, even with cause, because it makes them a very exhausting person to be around.
Oh, and for more specific pet peeves:
-Getting straight As, and constantly working, and then leaving an exam hall saying "OMG I FAILED!LOLOLOL".
I don't think I need to explain why that behaviour is incredibly obnoxious and rude.
-Never looking a person in the eyes when they are talking to you(disregarding genuine nervous predispositions/conditions)
It's just rude. It just seems disrespectful, at which point, I'd leave the conversation as it looks like that person is deliberately going out of their way to ignore me when I am talking to them(if I said something rude, fine, but I usually gauge that well, I mean, on no provocation: saying "Hi!", with them responding, and as you talk to them.)
-People who say "you weirdo/no life," etc.
Yes, because you are the social paragon on what is a social life and what isn't? Also, of course, playing Call of Duty is way more normal than someone who enjoys Scott Pilgrim Vs. The World, or Persona 4.
I think that's about it for now.
To be honest, there are very few things that would cause me to actively hate a person: I usually gauge fairly quickly if I'm not wanted, if I am disliked and if I really want to know that person.
What I hate the most is when people feign threats because they find that "hilarious", or when they put people on the spot and ask them personal questions because they enjoy seeing a person get embarrassed. Also any sort of person who sees women as objects, fuck those guys, active homophobes, racists, people who spread rumours, and just general predatory behaviours.
Yeah, suffice to say, a lot of what I just said has been influenced by my experiences in secondary school.