For each Fallout game, is it possible to do a pacifist run? (A pacifist run is a playthrough where you complete the game without killing anything.)

If you don't count Horrigan and The Master then yes, they are possible, but very hard

And even then.

The Master can be talked with, assuming lots of talking skills and .. was it science.. and all info from the glow
Horrigan can be handled by turrets and that team over there... with luck.

Here is the wiki page about that, but I for one know that in F2 you don't need to kill President at all, so that is wrong, but as I know the rest is correct! Try it if you will, but it's very, very hard and in games like Fallout when and where someone really, I mean really deserves to die I think it's Utopic! ;-)

[url=http://fallout.wikia.com/wiki/Forum:Completing_Fallout_without_killing_anyone]http://fallout.wikia.com/wiki/Forum:Completing_Fallout_without_killing_anyone[/url]