Heinrich Müller is a scientist who kills and tortures people for his experiments, and his own amusement. examples of atrocities he's committed are:

Forcing Alan to watch a VHS tape called "Felix yiff yiff yiff" while he holds holds his eyes open

Springlocking Jim, keeping him alive, and then removing the springlocks from his body with no anesthetic

replacing Billy's skeleton with a T-800 style endoskeleton, robbing him of touch, smell, and feel

sending Jim to hell, watching him crawl out of hell, then kicking him back down, because he finds it interesting the lengths someone will go to stay alive

beating the shit out of Alan's wife, right in front of him

surgically removing the organs of people, and keeping them conscious with no bodies

turning people into cyborgs. these cyborgs can feel the cold hard metal piercing their skin

and finally, turning the Mario guy into a living generator, and using him to power his lab.

he uses souls, organs and blood in his gruesome experiments, which involve trying to recreate life artificially, and to achieve the ultimate goal of immortality. the only people he likes are David, Wilhelm, and Lil Timmy