About halfway through the game, you meet Sander Cohen, this crazy artist dude. When you finish the level, you can just leave him or kill him. If you kill him at that point, you can't get one of the Achievements "Find Cohen's Room." A while later you will stumble onto Cohen's apartment. If he's alive, two Splicers will be dancing in the main room, with Cohen watching from his locked private room. Kill the Splicers and then Cohen comes out to fight you, thus unlocking the door and allowing you to earn the Achievement.