With such a broad question, you'll no doubt receive generalized answers....
For me personally, I enjoy rape fantasy. The fascination could be the pure taboo of it. Could be the loss of all control. Could be that even though we all know and appreciate the importance of safety and limits and soforth, rape (not a set-up play rape) is NOT safe and it's not pretty or controlled or set up in any way. It's ugly and raw and terrifying and those things have a draw.
Do I advocate rape? Absolutely NOT. This (for me) is a fantasy only. I'm not sure if I could handle a play-rape; I've never tried.