Oddly, I've never played a Deus Ex game where I was a killer of any kind. I was pacifistic back in the original Deus Ex (which I recommend everyone play, it remains to be one of my favorite games) and I was pacifistic in pretty much every one of my Deus Ex HR files (none of which I beat, but one day). For those interested, since this wasn't a big thing, they actually reintroduced the pacifistic run playstyle back in Deus Ex: Human Revolution - Director's Cut, where you don't have to kill bosses, so that was a nice change.
Heck, even in Dishonored I immediately- inherently- decided I wasn't going to kill anyone. It's mainly because you're rarely given the choice to do something like that so, when you are, it becomes both a necessity and a challenge for me. After all, I've never really been the type to kill absolutely everyone around; I think even in games that didn't support it I would still try to find a way every now and again.