Wow...um... It's hard to pick...
I HATED Miranda from "Mass Effect 2" within seconds. Special treatment twat...
...Yet, I'll go with Haseo from DotHack//G.U. Yeah, he grew the hell up later on in the trilogy, but at first I wanted to wring his damn neck. He taught us all that asshole characters, though fun to watch tear people apart, should NEVER be introduced to emotionally introverted (and obviously socially disrespected) teenage girls who just wants to escape from her shitty lives. I wanted him dead immediately.