"Clean Hands" glitched?

This achievement seems to be glitched for a lot of people. Make sure you play the game from start to finish. Using mission select does not seem to work.

Keep in mind, though i can't guarantee this, but if you kill any of the assasin's in the prologue, when you get back, they apparently count against you, so you hafta basically run away until they decide to kill the empress.

I can confirm that what cosplaykid21 said is true. I completed a painstakingly slow playthrough of Mission 1 having killed no one in the prison or sewers. I got the Vanished achievement, but the tally showed 2 hostiles killed. Was stumped until I realized I killed both attackers in prologue, which I had not done in my first game.