All of the "research volunteers" are cancer patients.Shouty Guy's madness has driven him to the point that he believes people with cancer are "the cancer" themselves. So he kidnaps them/tricks them into coming to his facility, puts them through meaningless and disturbingly lonely tests (where they can die on their own), yells at them, and then, after they've survived the mental scarring, shoots and kills them.
WMG / Mondo Medicals
Monk), he eventually starts up his own cancer research center despite having no actual medical training. Only truly desperate people dared to work for him, even moreso once he implemented his "dissorientations" to test his potential employees. The combination of his unresolved issues and the increasingly bizarre people he wound up working with drove him progressively more and insane until, by the time the game begins, he has completely flipped, seeing other people as cancerous growths. That's why he's always seen standing in the middle of an empty room — there's no one else around because he shot them all.