User blog comment:Noctosphere/We finally get a Human as a big bad. Your thought?/@comment-74.58.154.192-20160602204901/@comment-34326521-20160604110902

Toni is going to go rogue, but in a different way. She's going to be the new Castiel, in a way.

As for reality, this is fiction. Vampires, et al., don't exist in reality either. Besides, there have been a few human villains, and humans who did evil things. Hansel, the guys who enjoyed hunting humans, Bela, the guy who put on silver claws and started killing monsters, the guy who tried to eat Sam to gain Gadreel's healing powers... Of course, these aren't the focus of the series. If monsters are the villains, humans are generally the victims. We rarely see monster-on-monster interaction, although that would be awesome.