Pacifist

Pacifist is an achievement/trophy in Deus Ex: Human Revolution. It is earned by completing the game without anyone dying by your hand. This excludes boss fights, which do not give you a choice about killing the antagonist.

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 the boss, and the boss only, is permitted while preserving the ability to obtain this achievement. Killing enemy soldiers during a boss fight will lock out this achievement.

The use of weapons will NOT lock out the achievement, if the enemy survives. This can be used to briefly stun (recommended) armored enemies in order to move past them when combat is unavoidable.

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.

Unfortunately if you lock out the achievement by accident, unless you are 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.

It is completely possible to complete all side quests in the game and still receive this achievement.

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.
 * Wall-punching should be done with extreme caution, as punching through a weak wall with the brief cut scene showing Adam breaking the neck of the character behind that wall will result in a kill, so keep an eye on your radar.
 * Notice the sparks emitting from the Arm Augmentation when attempting a wall-punch. If it gives off steam and visible white sparks, IMMEDIATELY reload, as this is a clear indication of someone being behind the wall.
 * Punching through a wall when an unconscious body is at the other side sometimes changes their state to dead, possibly locking out the pacifist achievement. When tested this happened in Derelict Row but not in Alice Gardens laundry room, so it does not appear to happen all the time.
 * 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).
 * Dragging an unconscious body down a long flight of stairs may result in a kill.
 * 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. However, said character may not always be registered as dead and will still allow the achievement to progress.
 * Turning the bots and turrets against enemies by hacking terminals may result in a kill.
 * Turrets, cameras, and bots do not count as kills; however, exploding bots may result in a kill. If the message “Self-destruct sequence initiated” played before explosion then any kills as a result of this event will not lock out pacifist achievement. Bots explosions killing enemies outside of self-destructing will count as a direct kill and prevent player from getting the pacifist achievement.
 * Not giving a weapon to Arie van Bruggen may result in a kill. (confirmed NOT, you can safely let him die).
 * Failing the dialogue challenge with Isaias Sandoval causes him to die, which may lock out Pacifist depending on the game's version.
 * Shooting a target in the head using a PAX-22 may result in a kill. Shooting the guard outside Double-T's apartment before and after he gives the warning cutscene results in a kill rather than a knockout. This also applies to the P.E.P.S., so be careful.
 * The knockback from a P.E.P.S. explosion can kill an enemy if it made them hit an object a certain distance away. A flying character that hits a standing character in this manner will kill them both, resulting in two kills even if the first one was tossed within the "safe distance". An object blown away by the P.E.P.S. explosion will kill an enemy if it hits them, so be very cautious when wielding this weapon.
 * Killing the insane workers at Panchaea will deny the player of Pacifist. This might pose a big problem to ill-prepared players, as the sheer number of hostiles makes it hard to go through an area without attracting their attention, which may cause the player to open fire and kill them.
 * During the final boss fight bring a laser gun to avoid the risk to kill the insane workers spawning later in the fight.
 * While taking out bosses lethally does not count against Pacifist, killing Pieter Burke in the Director's Cut version of the game denies the player the achievement, as he is counted as a standard enemy soldier by the game. DO NOT kill Burke.
 * Killing the Hyron Drone with the Trauma Kit in Rifleman Bank Station in the Director's Cut does not count against the Pacifist achievement.

Overall, it is best if major confrontations are avoided, if at all possible in order to save tranquilizer ammo or energy expelled from takedowns. Restrict fiddling with security devices to a maximum to prevent unnecessary casualties from bots and turrets. Unless a big fight (i.e bosses, bots,...) is planned in mind and well prepared for, lethal weaponry should not be picked up/equipped to avoid accidental discharges.

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