THIS IS A METRO : LAST LIGHT ACHIEVEMENT
During mission depot - towards the very end right after the cut-scene with anna and the two men talking. You'll spawn in front of a knocked out soldier holding a knife. As soon as you pick up the knife, you'll have the option to kill the soldier or spare his life and let him stay knock out. Hit X when the options come up to kill or knock him out, and you'll be awarded with a 10G achievement.