Simple. Don't attack Sander Cohen until after he has admired the Quadtych and opened the lock on the Medical Expert 2 Gene Tonic for you. After that you can attack & kill him to get the key to the Muse Box off of his corpse.
However, a better plan is to not attack Mr Cohen until you encounter him later on in his apartment in Mercury Suites. Waiting to kill him in his apartment will let you access an extra Power to the People machine, in his private room.