I didn't do a nonlethal playthough, but instead only killed when I felt the people were really fucking asking for it. A guard just doing his job? Cool, I got no problem with you other than the fact that you're in my way. Tranq Rifle, Stun, or avoid entirely. But when those Belltower fucks [spoiler]came crashing through the roof and slaughtered an entire hotel worth of innocent people?[/spoiler]
Yeah, press and hold Q, motherfuckers.
Oh, and I really wanted to kill that corrupt cop O'Malley fuckhead, because I know that it's possible for someone as connected as him to dodge any case put to him, regardless of how thorough. But I got him arrested instead because it was what Jenny wanted. Sort of was expecting him to be back when I came back to Detroit. If he had been, I would have put him down for good.
I think the only other people I killed were the Belltower guys going after [spoiler]Malik[/spoiler], both because they deserved it for being fucking assassins and honestly I'm not sure I [spoiler]could have saved her[/spoiler] using nonlethal methods.
The game gives up bonus exp for going nonlethal anyway, and there's basically no reason to ever hold Q unless you just want the satisfaction of a kill. Not that you need exp, at all.
OH! PROTIP! Getting emp shielding in the armor line makes you immune to fucking electricity. I didn't know this until the very end of the game! I would have got it a lot sooner otherwise!