The original Deus Ex game was similar. I remember getting about 2/3 of the way through without killing anyone. I remember being backed into a corner and finally making a kill, then "screw it, I'm killing everyone". That was an awesome game though, so many different ways to accomplish the mission. At one point you become locked into a single path though, which always bothered me - IE, you can't choose to continue helping the bad guys at a certain point, you always end up fighting against them.