Dr. J.S. Steinman
Dr. Steinman is a high-profile cosmetic surgeon in Rapture. Advertisements for his services are plentiful, and his office is the largest and best-equipped in the Medical Pavilion. His fixation on physical perfection--combined with the effects of increased use of Adam--turn him from a surgical perfectionist into an "artistic" monster. His audio diaries, as they descend further and further into insanity, are the first real exposure the player has to the total effects (mental and physical) of repeated splicing.
When Fontaine's recruits rose up, Steinman locked himself in the surgical area of the Medical Pavilion. Atlas informs Jack that he'll need the key Steinman possesses to travel to Neptune's Bounty.
Fighting Dr. Steinman
Dr. Steinman is a relatively easy fight as he will simply attempt to shoot the player with a machine gun. There are several different ways to go about winning the fight. The most straightforward, and probably hardest is to simply shoot him to death, it takes a while to whittle down his health. One problem with this strategy is that once he is at low health he will run over to a nearby first aid station and heal. Hacking the first aid station will set it to poison him when he uses it and help finish the battle more quickly.
Other strategies include using a low area containing water to shock him, and using the telekinesis plasmid to pick up and throw a nearby barrel at his head (This is possibly the easiest method).
There are also a nearby security drone and turret both of which can be hacked before the battle begins. The turret isn't very useful since it rarely has line of sight to Dr. Steinman, but the drone provides some extra damage and hacking the turret will ensure that there is one less thing is shooting the player.