For single player games where I'm dying a lot, or just having a hard time:
"You're failing to entertain!! STOP FAILING!!"
"JUST F***ING DIE ALREADY!!!!"
The latter I've used recently while playing Castlevania: Symphony of the Night while fighting those guardians, and the giant lightening scepter wielding boss.
For PvP in WoW after some douche is killing vendors, quest NPCs, and so on, and I can't kill him:
"Hope you get hacked and never recover!"
Thinking about it now. Those aren't very mature things to say, even out of frustration.
/ashamed