Chaos is similar to a morality system found in other games. Actions such as murder, alerting guards, having bodies found, and all other disruptive (non-stealthy) actions will increase your “Chaos Level”. Lower chaos level leads to a more normal world environment; with less rats, plague and death. NPC’s actions will also be geared slightly more positive towards you.
Your chaos level is cumulative across all of the missions. It will go up and down based on how you approach an entire mission, in regards to the main target and optional objectives. If you choose to outright assassinate your main target you will greatly increase your total chaos level. Conversely, if you opt for the non-lethal alternative you will drastically lower your chaos level for that mission.
In order for the trophy to unlock you must finish the final mission with a total chaos level for the entire game that is “low”. It doesn’t matter if several missions are marked as “High Chaos” in the summary, as long as by the time you reach the final mission you have pushed it back into the “Low Chaos” range. It is ideal to this trophy in tandem with Clean Hands, because avoiding killing people altogether is the best possible way to maintain a low chaos level.