I wont say any spoilers. Or at least any plot-related spoilers. Mostly. But I will comment a bit on some things that made me thoughtful on this game.
So, I played it mostly non-lethal. Not because of achievements, but because, well... it felt right.
Fact: I'd say that roughtly, like, 10-20% of Dishonored characters are utterly disgusting people (I don't think it's a coincidence that low chaos gives you around that kill percentaje while still giving you the good ending), and about 70-80% more are on a scale from semi-disgusting to flawed. You find about ten percent nice people.
You have two "objetive" ways of judging this. One are documents and diaries you find. Another is the secrets the Heart tells you.
And the thing is, that you find that even amid lower class guards, not everyone is a monster.
You see, most of the time the heart tells you tactic stuff like "Careful, he's armed". And sometimes they tell you that this particular NPC is a murderer, or a rapist.
But sometimes you casually aim the heart at some guard or thug, and you get stuff like:
"He feeds a stray dog every night. He named her Billy".
Or
"When not at his post he searches for his sister, missing a week".
Or
"He visits his father in the asylum, even though the man no longer remembers him"
When you get stuff like that. And stuff like in the diaries, suddenly murdering people out of expedience, even in a computer game in which the main character is basically a hitman (!), gets distasteful. During the game I honestly felt bad about doing it, and in fact went waaay out of my way of killing NPCs. I wasn't a vigilante and wasn't about to judge every character I met, but in a pinch, unless the Heart told me the enemy NPC in question was fucking rotten, I preferred to avoid a kill and search for an alternate pathway, if possible.
It made me think. See, most people are gray, I'd say. There are very few truly shining examples, and far more dirty rotten ones. But few people truly have unredeeming qualities. For instance, one person from my workplace whom I regarded as a dick, recently did something very brave to try to help another person. Even though it got him in trouble, he did what was right. At a point that noone else was willing to do it. Really, he was the fucking unlikely hero of the day. Made me reconsider my own attitude.
I guess that my point is that while you have to be wary, and watch your back. But that even though very few people are really good, it's also true that very few people have absolutely no unredeeming qualities. Sometimes you get unlikely heroes. And sometimes you're the unlikely hero, I guess.
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As for the main characters in the game, when they were assasination targets, I tended to gravitate to non-lethal options because for the most part they felt either more pragmatic plotwise, and/or more satisfying
I think my most difficult moral choice was at a point in the game in which you had to choose to kill one of two main characters who were mostly morally bankrupt but which had both done you a good turn (or more) in the past. Not "choose a third option". (In the end I decided to spare the one I felt was the more interesting of the two). I would comment which characters I mean, and which I chose, but it'd be spoiling things.