I have a dimmer view of human nature. I'd say most of those fantasies fall under the “this would horrify me if it happened in real life and I was then caught and punished for it” category.
Then you really don't know very much about human nature. Lots of men are excited by fantasies involving rape and brutality toward women. That doesn't mean they'd commit such acts in real life if they knew they could escape punishment. The whole subculture of BDSM is about acting out these kinds of fantasies in the context of consensual role-playing.
Yes, because without the consensual role-playing part, there's a good chance they'd be doing hard time.
When those constraints are taken away, when such brutality is countenanced, a (not)surprisingly (to me) high number of people will gladly rape and stomp and worse. Or maybe, along with my lack of understanding of human nature, I imagined much of human history. You know, the Middle Passage, the Holocaust, the Rape of Nanking, the Rwandan Genocide, all of your garden variety sexual assaults, those recurring delusions. I guess it was all just role play and the participants forgot their leather, latex and ball gags.