High chaos is determined by how many enemies you kill...I believe you must kill 20% or more of the enemies in each mission to reach high chaos level. It is recommended you always kill your target, though not a requirement. High chaos will also effect quite a bit of other things in the game. These are just a few of the things I know for certain change throughout the game: Bloodfly infestations will become much more prevalent. The Grand Guard will be more numerous. Some dialogue and tone of characters you speak with in the game will change. And of course, the ending.