LOL, did you just call most women mentally ill? I work in psychiatric care/research. I've worked with rape victims. That isn't what I this post is about. Fantasy is just that, fantasy. I'm sure most people would agree that murder is bad. Yet, many video games allow it - heck, they encourage it. It's self-evident that rape is bad. I don't think I've ever seen a case where it was openly cheered or encouraged.
A lot of people probably don't know this, but the most common fantasy's for women involve power dynamics. Losing power. Gaining power. I'm sure most women wouldn't think of themselves as children, but the number of them that like being spanked is pretty high. Ever met someone that has been bullied? What do you think their fantasy might be? In many cases it's gaining power over their aggressors. It's a power dynamic. Plus, women get turned on by male qualities of strength, decisiveness, taking charge, fighting for them, etc... Things that skirt some pretty dark areas (but no one likes to talk about).
Anyway, I don't think anyone disagrees with you. We should bury those people under the prison. But, that isn't what we're really talking about here. If you begin to fantasize and plan these things, yeah, that's a step too far. Just like if you staring thinking about shooting up your school after watching Platoon, the movie.