Don't kill Cohen after completing his "masterpiece". When you go to the Olympus Heights and get to the area with multiple levels (not Fontaine's house for the poor) to get the second half of the antidote, visit one of the apartments on the first level (to your right from the entrance). This is Cohen's apartment. When he tells you not the disturb the dancers, "knock over" the record player and deal with the upset duo. Cohen himself will then come down to kill you. He can be taken out then. Go upstairs in his apartment (the door to your right upon entering his place) to receive the trophy for entering his apartment and reaching a Power To The People machine. Now, Sander Cohen will not stay dead. Pickpocket his corpse for his Muse key. When you get back on the Bathysphere, go to Fort Frolic and access the Muse (the chained up display) for some decent items, especially if you need first aid kits. Using your chemical thrower (and perhaps anything else), walk to the "masterpiece" and add Napalm and Criticism together, then torch the awful thing. The area will turn red and Cohen will appear for round two. He is a little tougher to kill this time and fights like a Houdini splicer. Pickpocket him again for the same items as last time (including the key for the already opened Muse). He is now permanently dead.