Of course there arise times when you have to kill someone in a game. Part of the escapism of a roleplaying game is that your character can do things that you'd never do - or want to do - in real life. The vast majority of us would never wish to take someone's life in reality, but in a game this is a subject that can be approached in relative safety.
Slaying hordes of evil minions can make the death of an NPC at the hands of a PC into something trivial. But think about what your character would actually be feeling at the time. A character who is conflicted and wishes he didn't have to kill the NPC, and who mopes about it afterwards can be a more interesting and believable character.
The downside is that such a non-blasé approach to killing NPCs might make your fellow players think there's something weird about you.
R2-D2: Whoa, Annie, you just killed your own father.
Anakin: I have no father.
R2-D2: Seriously, time out. I think maybe you should take a break from the game.
Anakin: What? No, no, no. This is fun!
R2-D2: It is?
Anakin: Anakin is a deep and complex role. It's great practice for my character acting.
R2-D2: Not to be sexist or anything... But you seem totally out of control.
GM: Pete, it's okay. Annie has been discussing her character direction with me.
R2-D2: I mean, killing NPCs is fine, but you seem to be really enjoying it.
R2-D2: Not that you shouldn't enjoy it, but you've got to hide it when you do.
R2-D2: Um... yeah.
Anakin: Thank you Pete, for your unique insight into roleplaying.