The story wastes no time reminding you that this gory detail isn't intended (or at least, isn't solely intended) as a feast for the eyes of bloodthirsty desensitized gamers. Your first foes, as a politically motivated assassin, are the corrupt usurpers of power in a once honorable (hah!) regime. This of course means that the grunts in your way are going to be the police and soldiers of that empire, who believe it's their job to stop you. Perhaps some of these conscripts are as ill-intentioned as the bastard nobles writing their orders, but perhaps they think they're doing the right thing. On your first assassination mission, a friend of the revolution begs you to spare her uncle, a captain of the guard, who she insists is a good man trying to make changes from the inside. It's up to you to decide whether you give a shit, or whether you can afford to leave him alive. You may end up caught in a situation where he has you at gunpoint and you've no choice but to kill him. Average foes may furiously charge you after seeing a cohort stricken down, shouting "you've just made someone a widow!" These feel like real people - are you okay with killing a cop because you don't like the law?
If you aren't okay with that, then don't play that way. It may take a little lateral thinking and the occasional outright retreat, but the tools and skills are available to finish the game without taking a single life - not even those 'necessitated' by the story. Each situation can be approached with stealth or force, and may lend itself to one, the other, or neither. This is not a Splinter Cell: Chaos Theory or Mark of the Ninja, in which "you can go in guns blazing or stick to the shadows" equals "you're supposed to stay hidden, but if you're spotted you can (chaos) theoretically shoot your way out". There have been setups to which I started with a stealthy approach that was quickly abandoned when I simply couldn't figure out a way to progress. Nonlethal play isn't the correct approach or a challenge mode, it's simply an option left to the player.
The moment I realized that the game had been successful in rendering killing meaningful was when I met a gang of thugs trying to force their way into an old lady's home. Up until that point (about an hour into the game), I'd been mostly sticking to nonlethal takedowns on the guardsmen I'd faced (after I'd finished experimenting with the combat system). I started to prepare sleep darts to eliminate these three assholes, then I realized - fuck it. Why am I sparing a bunch of lowlife gangsters? I dropped from the roof - sword-first - onto the soon-to-be corpse of the first one, then slashed his punk friends to bits as they exclaimed "no one kills one of us!" Well, I did, and it felt damn worth it.