Okay, let's try and break this down.
Sexual fantasies are, by their very nature, transgressive. Yes, even the fluffy, romantic ones. As long as general culture remains negative about sex and sexuality in any form that isn't cishet procreative sex within the confines of matrimony with the woman not as an equal actor but an object sex is performed onto, this is going to remain true.
And the thing about fantasies is that our brains like to take the things we crave the most and mix them up with our fears, anxieties, pain, and trauma into a melange of, sometimes, truly epic levels of fuckery.
But here's the secret - things we fantasize about, from the most wholesome to the bizarre to seriously fucked up? They are very, very often NOT what we literally want.
Being into dubcon or noncon doesn't mean you actually want to be raped or rape. Being into monsters doesn't make you a zoophile. And fantasizing about violent, obsessive men doesn't mean you wouldn't run as far the fuck away from a man like that the second one of them set their sights on you.
If you're really interested in the subject, I recommend reading My Secret Garden by Nancy Friday, a compilation of anonymously submitted women's sexual fantasies. And, as it turns out, women fantasize about a lot of really violent, uncomfortable, and just plain screwed up stuff.
And, for most of them, even when they don't actively realize it, it's about reclamation. Of fear, of trauma, of loss of power. It's about THEMSELVES and how THEY feel. As weird as it's gonna sound, the men featured in those fantasies don't really matter, they're just a vessel, a manifestation of the extreme version of what you're dealing with and/or crave. A safe, cathartic way to experience something profoundly unsafe, unwise, and terrifying.
For women fantasizing about criminals, villains, monsters, and anti-heroes, it's very often about the idea that someone like that - intense, violent, with single-minded focus, and immense power - would love her, want her, always put her first, go against all his instincts/training for you without a second thought and be a clear and present danger to everyone but warmth and safety for her and only her, and burn the world itself down for hurting her in even the slightest of ways. It's a sexual version of the fantasy of having a pet tiger, one that would never, ever attack you or hurt you in any way.
And just like the people who want to boop the forbidden snoot, the women fantasizing about their fantasy Mafia Boss Lover are very well aware of the fact that 1) men like that don't actually exist, 2) the criminal world of their fantasy has all but nothing to do with reality, and 3) that the thing they're actually fantasizing about is being loved, wanted, and safe... just in a REALLY intense, exaggerated way. And, let's not mince words, there's also often a more or less strong D/s dynamics at play in the scenario, too.
Now, you can choose to be judgy bitches about it (goodness knows plenty of you in the replies, comments, and tags are), in which case I would suggest you examine why you're feeling such a profound need to shame women for enjoying themselves in their own little world, or you can apply the YKINMKATO mantra and understand that straight women, living in the constant state of preyhood, sometimes consciously or subconsciously reclaim power over that situation through transgressive sexual fantasies.
Also, fuck this idea that queer people only fantasize about healthy and wholesome relationships, romantic, sexual, or otherwise, as if at least half of Tumblr isn't simping for, oh, for example, Hannibal fucking Lecter. Do you have ANY idea how many Mafia and Thug BL content there is out there?! FFS, Tom of Finland, a WWII veteran who fought against Nazis, drew art of exaggeratedly masculine men in Nazi uniforms in pornographic situations as a way to dissociate himself from those traumas and fascists themselves as far back as the 1950s!
So yeah. Less judgement, and more taking some responsibility for curating your online experience if seeing someone's kink truly offends you this much.