I start off with good intentions. I genuinely want to play the "good guy" in whatever game i'm playing. Then something happens and I just can't do it anymore and random pixel people end up being shot/stabbed/blown up because i've been pushed over the edge by an irritating NPC.
Fallout 3. I coped when I met Moira Brown even though her voice grated on me like sandpaper on my testicles. I managed to cope with my patronising dad when I found him because hey, it's Liam Neeson, how can you stay mad at him? I even coped with the arsehole BoS when they proclaimed themselves my "saviour" and then proceeded to get their arses handed to them by a super mutant behemoth while I had to pull them out of the shit. But I couldn't cope with those little bastards in Little Lamplight. I tried, believe me I tried. A little gathering of children trying to keep safe in the wasteland but they were annoying as fuck, "open the gate little bastards", no? Okay then, off to download a mod and back I come on a minigun rampage, who's laughing now little shits, eh?
Then recently with Deus Ex. Pritchard just grates on me whenever he speaks. I tried to do a no kill playthrough. First I accidentally reprogram a turret to enemies instead of offline and it kills 4 guards. Then Pritchard starts with bullshit so I kill a few more people pretending they were him. Then I get to a point where I realise he's not such an arse so I kill more people for him acting like an arse for no reason.
In closing. I have no problem being the "bad guy", I do it quite a lot.
And killing pixels is fun.