Chernobylite (UP)

You'll eventually get a mission where there is mention of a traitor. The man in question is named Konstanty. He will be inside a building, where you need to drop through a hole in the floor. It's the main part of the mission to find him, so you can't miss it.

You'll end up with 2 options. Kill him or let him go. I chose the latter, as he is vital later on in recruiting Olga from what I've read around the Web. I dont know if killing him will unlock the achievement, but I'd not risk it of you also wish to recruit Olga.