How to Acquire
In either game, it is unlocked by finishing the game (excluding the prologue in Dishonored) without killing anyone.
A tweet from Arkane developer Harvey Smith confirmed that wolfhounds, rats, and river krusts do not count toward body total and therefore will not lock the achievement if killed. Killing or destroying other non-humans, such as fish, Blood Briars, gravehounds and Clockwork Soldiers will also not lock the achievement.
With the exception of the death of an unconscious character (which always counts as a kill, even if caused by, say, rats), Clean Hands does not consider Corvo Attano to have killed someone if a person is killed by someone other than Corvo. For instance if naturally-occurring rats kill a (still-conscious) City Watch Guard, it is not considered a kill, but using Devouring Swarm to kill a guard does count as a kill. Similarly, deaths resulting from a fight between guards and weepers, or thugs, are not counted as kills.
In Dishonored 2, a character killed whilst in a chokehold counts as a kill. Killing Paolo during the mission The Clockwork Mansion will not count as a kill, nor does killing him once during the mission Dust District.