Exactly what the description states - you must finish the game, start to finish in one run, without killing anyone. Chapter select does not for this trophy, so you must earn it one playthrough. That means no pistols, regular crossbow shots, assassinations or regular sword fights. If you must take out a guard or eliminate a main target, you must do so non-lethally, using either chokes, sleep darts or indirect means. There are more less-common ways of killing someone, and these may occur without you even knowing. The community lead at Bethesda compiled a list of ways that you can inadvertently kill someone (source) in Dishonored. It is really useful information that should be memorized, so to not accidentally kill someone during your Clean Hands playthrough. The information is as follows:
- Sleep Darting or Choking out an NPC and having them hit an object when falling, causing death
- Sleep Darting or Choking out an NPC near a ledge and having them fall to cause death
- Sleep Darting or Choking out an NPC and having them fall into water will cause drowning death
- Sleep Darting or Choking an NPC and allowing them to fall down a long flight of stairs will sometimes cause them to die (possible with the stairs in the Overseer building)
- Placing an unconscious NPC in water, or even near shallow puddles, can cause drowning death
- Placing an unconscious body on un-even terrain near water can sometimes lead to the NPC slowly moving and falling into water, which would count as an NPC kill (in Prison Sewers for example)
- Rendering an NPC unconscious near a damage source (such as a fireplace or lit grill) may cause the NPC to take damage and die
- Leaving an unconscious NPC in the middle of the street or alley, where rats have access to the body, can result in clean-up which will count as a kill for the player
- Accidentally casting Devouring Swarm near an unconscious NPC could cause the rats to clean up the body, counting as a NPC kill
- Accidentally casting Windblast near an unconscious NPC could cause damage leading to NPC death
- Having a Grenade, Springrazor or Whale Oil Battery explode near an unconscious NPC could cause splash damage resulting in NPC death
- Throwing an unconscious NPC into a Wall of Light, or near and Arc Pylon would incinerate the NPC and count as a kill
- Leaving unconscious NPCs in the area around the large door on which the player places the explosive device in Prison will cause the NPCs to die when the device goes off. This includes in and around the nearby dumpster and behind the consoles and gate switch across from the door on the upper level.
- Rendering an enemy unconscious while fighting a nearby Tallboy can cause the Tallboy to stomp at the player, essentially killing the nearby unconscious NPC in the process
- Rendering an enemy unconscious and having another enemy throw a projectile (such as fire bottle or grenade) at the player, can kill the nearby unconscious NPC
- Leaving an NPC unconscious in the Brothel Steam room may cause “bite” damage due to hagfish in the center pool, killing the NPC
To make this goal easier on yourself, avoid as much contact with the enemy as possible. Powers such as Blink 2 and Blend Time 2 really help in this regard, and you should not hesitate to use them.
Be sure to not kill the two assassins that attack you and the Empress at the start of the game. If you kill them, this trophy will not be obtainable.
Note that killing neutral creatures such as Rats, Hagfish and River Krusts do not count against you for this trophy, so if you need to kill any of those do not hesitate to do so.
Enemy Dogs, however, DO COUNT. Be sure you do not kill any dogs during this playthrough. Killing one will negate this trophy but will not count towards your "Kills" stat at the end of a mission, so just be aware of that. Do not kill Dogs.
When it comes time to eliminate a main target, you must do so by indirect means. These are pointed out to you in-game, but for a complete walkthrough of how to do these refer to the Poetic Justice trophy.
After completing each mission and arriving at the pub, visit Piero and replenish your sleep dart ammo. Sleep darts allow you take non-lethally take out an enemy quickly and from a safe distance, which is invaluable.
Make sure you save the game often. We cannot stress this enough. Dishonored allows you to save the game manually at any time, and then resume from that exact location should you need to. Use this to your advantage and save the game after clearing difficult section/completing a mission.
When you are presented with the states screen at the close of a mission, make sure that it states you have not killed anybody. If you have, replay that mission right then.
Once you have finished mission nine, without killing ANYONE, the trophy will unlock.