I've just beaten Deus Ex: Human Revolution and gotten the Pacifist achievement.
At the end of the game, Eliza offers you a choice of three different stories which you can provide to the world, and one where you don't broadcast anything. The no-broadcast option seems the most honest to me, and it's the one that I ordinarily would have chosen, except that the only way to keep word from eventually getting out is to murder everyone on board the station.
Staying true to my character, I chose the Darrow ending because it meant nobody had to die. Afterward, I started wondering if I still would have gotten Pacifist if I had chosen the murder-everyone option. Trouble is, I can't go back and check – once the achievement is unlocked, it's unlocked, no matter how many other playthroughs I do.
If I qualified for the Pacifist achievement up to and including the final boss, but then chose to murder everyone on the station, would I still have gotten Pacifist?
Best Answer
You should be granted the 'Pacifist' achievement, along with any difficulty related achievement at the point where you've defeated the Hyron Project encounter.
This is before you make the choice at the end of the game so the choice you will make will not reflect on getting this achievement.
It is worth noting that you can save the game after killing the last boss, and before selecting an ending - as a quick method of viewing each of the endings.