I love how one of the main storytelling concepts of this game is largely making the criminals more noble than the aristocrats. Slackjaw and Daud, for example, are far more trustworthy and honorable than the "good" people in society like the Pendletons or Campbell. That being said, though, the game still goes to great lengths to really blur the line of morality. Even the Lord Regent, with his OCD and obsession for order, thought that the destruction of the Empire was imminent unless he took action; that he alone could save it. I would still consider him evil (literally started the rat plague) but his evil is understandable from a character point of view.

Would Havelock, Martin, and Treavor Pendleton all be considered evil? They betrayed you and tried to usurp power, but they all have redeeming qualities as well. Honestly, the world of Dishonored seems to be so brutal and twisted that most of the characters are messed up in some way. But which would you consider truly evil?

The term 'evil' is a sledgehammer applied to a job better suited to the scalpel.

Dishonored is very grey, which is why this matter is subjective. That's why the question is "who would you consider evil?"