How much violence you want to commit in Mark of the Ninja is pretty much up to you, so far there is no one you actually have to kill, in fact you can if you want try to sneak past everyone. Seemed like a fun time so I tried it here and it worked pretty well for me. In fact I found it a little bit faster since my priority was getting past guards without being spotted rather than killing every last one of them via stealth kills.
Interestingly enough you actually forgo honour if you try this route, a lot of the seals in this game are based around enemy kills. I guess you are a ninja after all, you are meant to be an assassin not a just a thief or a spy.
Later on you unlock costumes which let you tailor your play a bit more based on what you enjoy, but I will leave off talking more about it until later.