Chaos is the system Dishonored uses to determine the decisions the characters make for you in the game. Kill more people you have a high chaos level resulting in more negative effects & vice versa. I've only realised this after completing the High Overseer mission. I don't really want to start the game again so will continue playing the remaining missions trying to get a low chaos level to balance out the earlier missions.
If I had the time I would carry on playing the way I am & then play the game again assuming it has a new game plus option to go for the low chaos ending.