Fridge Brilliance: At the end, the Silver Age villains who formed the "Mankind Liberation League" to exploit the superhuman conflict receive a bout of Laser-Guided Karma by being forced to work in Bruce Wayne's hospital at the end. Notice, however, that a further level of of Karma is active in what duties they've been assigned; would-be world conquerors and mass-murderers Lex Luthor and Lord Naga are assigned demeaning roles like mopping floors and emptying chamber pots, while relatively low-level thieves like the Riddler and Catwoman get relatively minor jobs.
Another, not mutually exclusive possibility: those with some degree of compassion (Catwoman) or at least no desire to cause harm to innocents (Riddler) work with patients, while the murderous ones are given jobs that don't put vulnerable people within their reach. In particular, the Riddler is seen dealing with a small child, a role that someone of his talents would probably find about the right audience.
At one point, as covered under Strawman Has a Point on the YMMV page, the Nineties Anti-Hero characters imprisoned by the Justice League complain that killing was the only way to stop threats like Genosyde. However, a previous issue has Batman mention that Genosyde blew up Arkham Asylum, Blackgate Penitentiary, and Belle Reve, the three most prominent supervillain prisons in the DC Universe, killing almost all of Batman's villains (and scorews of villains in general, one presumes). This would make Genosyde more like an especially extreme example of the "newblood" vigilantes, who have already been described as spending more time fighting amongst themselves than anything else.
Blackgate is not a supervillain prison, but a regular jail, so thousands of normal inmates were killed. Arkham is famous for its super-powered wing, but it's still just a sanitarium, and plenty of normal people, including out-patients, were probably there, too.