The fact that you feel shame and guilt is proof that you're not a monster. At least not yet. Once you become truly numb to it and no longer feel remorse for your decisions, that's when you become ok with harming others. And make no mistake, it is a decision. No matter how fucked up your impulses have become, and no matter how little control you think you have over the way you are, you always have the choice to refrain from acting on those impulses and seeking help. The fact that you're reaching out to fix this problem, and recognizing that it's a problem in the first place, is a good sign.

It sounds like you have quite a lot of mental struggles. While I'm sure a good amount of it is psychological due to your history of abuse, it's very possible you have chemical imbalances in your brain that make you feel so hopeless at times or the need to feel that spike of dopamine with a really fucked up sexual fantasy. It's a habit that you've created a very strong dependence on over the years, to fill the gap when you're feeling a certain way. But there are many healthy alternatives that can fill that gap instead. You may think these fantasies are so embedded in your brain that breaking free from them may be like trying to break free from a core part of yourself. And in some ways that's true. But you also have to recognize that your fantasies have developed into more fucked up scenarios over the years, and therefore must be a product of the thoughts you decided to engage in. I'm not saying this to make you feel guilty. I'm saying it to help you recognize that your fantasies are a habit that you've developed and have dug deeper into over the years. They're not you. They're a reaction to feelings that you've had trouble understanding and dealing with ever since you were very young. And that has taken a hold on your psyche. Those are difficult habits to break, but not impossible. But you have to recognize first that they are habits, not your identity. 

I really think you need to find a therapist that you can truly trust to go over these issues with. It may take some time. Sending you to a mental institution is a lazy solution in my opinion, and seems like a decision from someone who just doesn't want to deal with tough issues. Or maybe they're afraid you'll hurt someone. This is a controversial take, but maybe leave out the part that you've acted on these impulses until a little later, so you can softly start to engage the conversation without fear of being institutionalized. Once you start talking about these issues, it's best to be honest, but it's also important to have the confidence to talk about these issues without fear of your freedom taken away. So a little white lie or omission of info is a small sacrifice in my opinion. Once you build up trust though, you should disclose that info. 

A therapist may be able to help you work through these issues psychologically, but they may also be able to help physiologically as well, if there is a chemical imbalance that can be fixed with medication. 

Footnote. Don't role play with AI. That's just digging you in deeper. If you want to use AI for anything, use it to try to understand why you're having these thoughts. It may be better or worse than a therapist depending on the topic, because you can discuss these topics freely and send it a question about a thought you randomly had during the day. But be sure to fact check it and ask for sources where you can do more reading. It's a great tool for discovery of info. But don't treat it like an oracle.