This achievement is obtainable by completing the Kelley storyline.
It is obtained after the mission: Falling From Grace
The last mission you have to do to get this is Full Of Grace or something along them lines. In this mission you infiltrate the house of where the slaves are kept. At the end of the mission you should get a cutscene where Kaitlin Lau 'arrests' Mary Kelley but lets the slaves beat her to death. After that cutscene the achievement should pop.
Please leave comments/replies incase I did anything wrong. Feedback is appreciated :)