alt Hacker NewsPostal 2 can be played through without ever hitting anyone, which I've done for fun. Although it's the exact opposite of what you're supposed to do there, which is the joke.
Haven't been able to repeat that in Deus Ex, you can't play through the campaign as a total pacifist, and Carter explains as much right at the beginning.
I vaguely remember doing a self-challenge pacifist run... I think I had to move all the unconscious bodies on the PCRS Wall Cloud up to the dock, because otherwise, I would be indirectly responsible if it sank while they were unable to evacuate.