Generally I hate 10 year olds, but not just because they're young. I'm only 14 myself. I hate the ones that find it necessary to just blurt random shit into their mics in the lobby and during the games, the ones that never shut up. I don't mind 10 year olds in general but if you do something like that then you're just asking for trouble.
I hate British people that, as soon as you talk and reveal to them that you have an American accent, start shouting shit like "Fuck America!" or "All Americans are assholes that must die". I play Halo 3 and CoD4 all the time and there are just mobs of them that do nothing but talk shit about America, they don't realize that not all of us are assholes. But I don't hate British people in general, just ones that think that they're better than everyone, the ones that like to hear themselves talk, and the ones that stereotype Americans.
I hate Rascists. It's just so unnecessary to think like that. Those people are unnecessary. I personally think Racism is an illness. Something must be wrong with your head if you think that you're better than someone just because their skin is a different color than yours, or they talk or think or act differently than you, or they have different beliefs. We're all the same on the inside, if you cut a white dude open and a black dude open the same organs will spill out, and the same color blood.
I hate people who team kill for the fun of it. They shouldn't be allowed to buy the game because they're just going to kill people on their team instead of actually playing the game. They're not in it for the game they're in it to piss people off, so it's a waste of their money and everyone's time.
I hate campers, people who use cheap tactics or weapons, glitchers, and hackers. They play that way simply because they don't have skill. So they must cheat to win, or use an overpowered gun just to get a few kills or they use something like Martyrdom in CoD4 and then they just rush a room full of guys and get killed on purpose just so they can kill all of those guys with a martyr grenade, because they don't have enough skill with a gun. There are people who use Juggernaut in CoD4 because they aren't smart enough to keep themselves alive. There are dudes in Halo 3 whose first instinct is to throw a grenade because they can't rely on their shooting ability to get a kill. And the hackers who have to screw with the game to get ahead of everyone else, some think it's a display of skill or power or "leetness". Really they're just cowards.
And I hate guys that go into Rumble pit on Halo 3 or a free-for-all match in CoD4 with friends/guests and they don't kill each other. They just sit there double teaming everyone.
Last but not least I hate people who talk trash about legit players or people on their own teams. I especially hate it when people give you a whole bunch of shit for beating them legitimately, just because they're not as good as you.