This guy is a nut job and I really want to get at that chest in the container behind him in Fort Frolic. I know he has the key for it so I feel no guilt in killing him, but there must be a catch. They wouldn't give you the option to let him live if there was something to gain from it. Is there any reason to leave Sander Cohen alive?
I wouldn't recommend you I'll him for 2 reasons. There is nothing major to gain in regard to the story for keeping him alive, but if you keep him alive you will gain access to a power to the people machine later on in the game. The chest contains some money and I think it also has some splicer hearts. The power to the people machine is much more valuable. You also get a trophy/achievement for gaining access to his apartment. Once you get to his apartment you can kill him, so I would hold off for the moment, get the power to the people upgrade and then kill him.