You will meet Sander Cohen about halfway through the game, a crazy artist dude. When you complete the level, you can just leave him or kill him. If you kill him at that point, you can't get one of the trophies; "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 trophy.