To unlock this achievement you must avoid killing anyone (excluding Runt) throughout the entire main story. The easiest way to do this is by finishing off your opponents with fists or a blunt weapon, ensuring that they are knocked out rather than killed. Be aware that blunt weapons with high damage ratings will sometimes result in death. It is therefore best to use a low level weapon or your fists. During large fights where you have allies, allow them to finish off any opponents so that you do not deal the killing blow. Use your speech skills wherever possible to avoid conflict.
To ensure that you haven’t accidentally killed anyone, you can monitor your progress by navigating to: Player > Statistics > Combat > Enemies/People Killed. Note that this will show as '1' when you have killed Runt during the main story.