Given that Bioshock takes place in an underwater utopia turned dystopia, almost everyone in it could be deemed twisted, as most people didn't take the whole "trapped miles underwater without escape" thing very well.
Of this collection of miscreants, the most macabre has to be J.S. Steinman.
A renowned surgeon who specialised in cosmetic surgery, Steinman accepted his invitation to the underwater land of Rapture in order to see if he could further revolutionise the field while surrounded by the world's supposed greatest minds.
What actually happened is that he would instead get addicted to ADAM, a chemical substance that allowed the user to alter their genetic makeup, which had the incidental side effect of driving him insane. Still obsessed with plastic surgery, Steinman would decide to focus on creating the perfect looking person... on unwilling subjects.
By the time you find Stein, he's murdered countless people in his drug-addled attempts to alter people's appearances.