Pacifist

Pacifist is an achievement/trophy in Deus Ex: Human Revolution. It is earned by completing the game without anyone dying by your hand. (Excluding boss fights, which don't count toward this.)

Overview
The aim is to complete the game without killing anyone directly, or someone dying as a result of an action taken by the player. The  only  exception to this is the Boss fights where killing them is permitted while preserving the ability to obtain this achievement.

This achievement starts from the first moment of the game, when Adam Jensen is not yet augmented; killing anyone in the Prologue chapter will lock out this achievement. (This is rumored to be a bug, and will supposedly be fixed in a patch.)

Unfortunately if you lock out the achievement by accident, unless you aware that you did so and reload an old save, there is no indication that you have done so and may end up playing through the game and not get the achievement due to killing a character earlier in the game.

Strategy
Obtaining this achievement is not as hard it sounds; even in Hard difficulty it is quite straight forward. The important thing that is you need to check every single body that you take down to ensure that they are only passed out (indicated by the "zZ" icon in the HUD which appears when looking at the body), rather than dead (indicated by the Skull icon). There appear to be certain situations where you will receive the "Merciful Soul" notification, but the target will show a Skull icon upon inspection; additionally, moving bodies which have been unconscious for some time will result in them changing from "Zz" to "Skull," but neither situation impacts obtaining Pacifist.

There will be times where kills may occur by accident, including: If you kill someone you can reload from the last save (assuming that that save file doesn't contain a situation where you've locked out the achievement previously). So if you're going for this achievement it is important to save often.
 * Punching through a weak wall with the brief cut scene showing Adam Jensen breaking the neck of a character behind that wall will result in a kill.
 * Pushing a character off a ledge onto the floor below may result in a kill if done above a certain height (0.5 stories or higher).
 * Pushing a character onto a proximity mine (resulting in its detonation) will result in a kill.
 * Be careful when using non-lethal takedowns on opponents near hazards (e.g. electrified water, poison gas), as the knocked out opponent may fall into the hazard and die.
 * Dropping a character to his death in order to secure an achievement will result in a kill.
 * Turning the bots and turrets against enemies by hacking terminals may result in a kill.
 * Exploding bots may result in a kill. (does not affect achievement)
 * Not giving a weapon to van Bruggen may result in a kill.
 * It may be strange, but shooting target in the head from PAX-22 may result in a kill.

In larger areas it is better to avoid confrontation if at all possible in order to save tranquilizer ammo or energy expelled from takedowns.

Investing in advanced Stealth augmentation upgrades will also help in moving around.