Kelbear said:
1) Have you ever experienced guilt in a game? What was the situation?
2) Why did you do it?
3) Why do you think you felt guilty about doing it?
Most importantly,
4) If you ran into the same situation in another game, would you do it again?
After all these years of gaming, tell us about the situations that forced you to reflect on them afterwards.
1) Yes, especially in Deus Ex when the father in the French cafe tells you about his son joining a militia group and gives you the details of their security system. If you speak to him again, he says "I have given you my son, what more do you want?!" and if you speak to the mother she'll tearfully describe the son and beg you not to kill him. Also, right near the start of the game if you sneak up and listen to two terrorists chatting, they'll talk about the morality of the current situation, one even arguing for an alternative that will result in less bloodshed, which made me feel bad about killing him and his buddies.
Also in Vampire - The Masquerade: Bloodlines when I'm given the mission to kill Julius, a Thin Blood vampire, because he's been blabbing about vampires to a writer (since nobody bothered to tell him you're not allowed to do that).
To some degree in Fallout 3 when betraying various 'good' characters, although most of the characters aren't as easy to relate to, so it's not so strong there.
When setting Harold on fire in Fallout 3.
2) To explore all possible paths in the game, and to experience the emotions that go along with it.
3) It always makes me feel bad when I do nasty things to nice characters, ones the game makes me care about, that have personality and uniqueness, and when they or others are obviously upset at what I'm doing.
If they're nameless, faceless characters who simply wander around the world, I don't care about them, like the generic people in Fallout 3 or Bloodlines.
4) I would, but I always reload from a save and take the 'good' alternative afterwards.
Very rarely do I play an 'evil' character all the way through, except once in Fallout 3 where I played the whole game through as a total ***** (except where necessary to get extra quests).
I still played it through as a good guy as well though.
Deus Ex, Bloodlines, and Fallout 3 are some of my favourite games of all time, largely because they actually made me care about the characters.
I'd feel bad when Paul got mad at me in Deus Ex, and when I had to betray my fellow vampires in Bloodlines.
I'd feel good when I rescued hostages, or got out of a situation where I ensured that everyone survived.
When I play Deus Ex I typically try not to kill ANYONE - I'll run around with my tranquiliser dart gun and police baton and knock all the terrorists out. Then just for kicks and for the extra challenge I'll also go knock out all the civilians and friendly military.

I tried several times to intervene in the firefight between UNATCO and the NSF so nobody died, but it's pretty much impossible, no matter how many gas grenades I use.
Of course, I also occasionally save my game and then just run around the level slaughtering everyone, then reload the save.