The higher the level of Chaos, the darker the game experience. Your actions affect the city. A high number of deaths results in more rats and weepers, different reactions from your allies, and darker final outcomes.
The overall level of chaos is affected by the number of people Corvo kills in the course of his missions. The game can be played either stealthily, including the possibility of killing absolutely no one, or bloodily, killing everyone who gets in the way.