Dishonored judges how ruthless you've been with an end-of-mission stat called Chaos. It affects a lot about the way the game works out for you, but the game itself doesn't fully explain how it works. I misunderstood it, and accidentally committed myself to a darker path than I wanted.
The game also rates you on two entirely optional criteria: whether you were detected, and whether you killed anyone. For the most part it's obvious how to avoid these, but there are some subtleties to how they're calculated that could cause you to fail them without realising.
Happily, Bethesda sent us all the juicy details of how Chaos, detection and kills are calculated, so we can tell you.
What caught me out with Chaos was the fact that the end of mission screen calls it "Chaos (current total)". I got 'High' after my first mission, and since it's a total, I thought it was already too late to ever get 'Low', so I didn't try to avoid killing after that.
It's not. If you get High Chaos on the first mission but then don't kill many people on the next few, it'll go back to Low. In other words, redemption is possible.
But it's still not that easy to get Low Chaos. Here's what Bethesda say about how it's calculated and what it means:
Chaos is a value that is adjusted according to the actions of your character during gameplay. This system is a hidden mechanic and you will only see the Chaos rating displayed during the 'end mission' Stats screen.
The basic rule is killing less than 20% of the characters in a Mission should allow the Low Chaos rating to be sustained.
Your character's Chaos level will change the story outcome and lead to other various differences throughout gameplay, such as more enemy characters, more rats, or different scenes/environment items and conversations.
Kills by Rewired traps will contribute to player kill amounts and Chaos; that's Watchtowers, Arc Pylons, and Wall of Lights.
Rats, Hagfish and River Krust do not raise Chaos if killed. They also do not count towards detection of your character.
Wolfhound kills do not count towards Chaos, but they can detect your character. Wolfhounds can discover bodies, and their corpses will count towards “bodies found”.
Weepers do count for detection. They do raise the Chaos level if they are killed.
The best method for regaining a Low Chaos rating is to not kill enemy characters and take stealth routes to avoid combat when playing through a mission.
There are scenarios where performing a good deed will drop the Chaos rating by a small amount (Example: saving a character that is in distress).
Following “Non-lethal” objectives will decrease chaos by a significant amount when the mission is completed.
Not hiding bodies well enough after choking them out or sleep darting them can sometimes lead to other characters finding them or may lead to nearby rats cleaning them up, counting as a kill towards your character's stats.
Unconscious characters won't survive a fall from a great height, or a slip into the water which will also result in a kill towards your character stats.
All characters and creatures count as detecting the player character, with the exception of friendly characters, Rats, River Krust, Hagfish and Watchtowers. This is evidenced by the red bolt icons upon detection.
Friendly characters will turn into a detection if they see your character do anything aggressive/hostile (kill someone, be hostile towards the NPC, carry a dead/subdued body, etc.). They will gain the red bolts when this happens.