Dishonored: Not killing isn’t easy.
Dishonored hates you. Everything it gives you can be used to end life in a huge variety of interesting and entertaining ways, then it chastises you for it. You want to be a nutcase: chopping of exaggerated limbs and throwing their decapitated heads at their friends? Go for it. But be ready to face the consequences, which are dire.
Not only will the ending be darker, but the streets become more dangerous, riddled with more swarms of rats and a higher presence of Weepers (read plague victims turned shuffling, zombie-like shells of men). The game has no boss fights, just a constant fight with yourself, trying to hold back your barely contained aggression.
The problem is that the actual killing is better than many games that are focused purely on killing. People say power corrupts, and the game actually makes you question your own personality. If I had powers I would be a bad person indeed. At the heart of the story, you are performing assassinations for a group of people you don’t really know all that well, performing various acts of violence, without ever questioning their motives; blindly following orders. But there is always a choice.