What I like about games is the fact that I'm the most saintlike goodness there is. And my enemies are the most evil bastards imaginable. At the same time the 'good guys' are being depicted as a group, just like the 'bad guys' are. This way, I can 'kill' a bunch of really evil guys (AKA Allies versus Nazi-Germany, Some guy out of a dungeon versus Daedroth, Nanosuit guy versus evil Koreans and aliens). I like the game even more when it's not photorealistic (think cartoony, bad graphics, funny glitches), it reminds me I'm doing something that isn't real.
When games get better graphics, and the big fat frickin' thick line between good and evil is being blurred, and I'm going to do things I do not like. I will simply refuse to do them. I like killing people in Borderlands, they give me XP, so I can get some of those new skills, use some better cell-shaded guns. I do not like to kill harmless accurately mathematical depictions of humans, subsurface scattering, waving hair, actual emotions.
Not going to add to the Childporn/Warcrime discussing here. But I think the point was that they're both pretty offensive and disgusting. When I see those things (childporn/warcrimes) on TV, they make me sick. If I see them in movies (childporn/warcrimes), my stomach twitches. I do not know what kind of effect it (talking about warcrimes, not childporn here) has on me in an interactive media, but I'm not interested in finding out what it does to me. Most of the time a game makes me 'feel good' while playing, I'm saving the world, or I'm evil in this fantasy land. I also think the comparison between a game like GTA and MW2 is not right. I do not have concrete arguments for this. It just feels different for me.
That's why IW put a 'do you want to play this or not?' function in the game. Everybody knows what's going to happen, if you don't like it, don't do it. It's like renting a movie, you could watch that great new psychological thriller/horror and get the crap scared out of you a couple of times. Or you can rent that new comedy, which will get a couple of laughs out of you.