I actually considered something like this! One thing that fascinates me is how looks can mean so much, for instance with Ted Bundy many people were simply attracted to him and so a twisted serial killer became a tortured object of desire. There are also cases of women marrying serial killers in prison as they're attracted to them (I always remember Richard Ramirez being a prime example for this), and it seems to me like those people would likely be very into rough sex and that kind of thing, although you may want to avoid that in your EPQ if you're not comfortable discussing it.
As far as I know, there are pretty much no limits as long as you discuss the topic in an appropriate way and you're guided by the facts. Other topics I considered were pedophilia and sex education (the former I did eventually decide was too emotive while the latter just fell through, so I'm now planning an artefact hopefully). Ask your supervisor, but I think this sounds like a great idea!