I think part of the problem is that murder scenes are short and rarely show much real pain. Writers sometimes show too much of a rape scene (and possibly do this for male titillation). (Even the scene leading up to a rape is generally more than I want to watch)

Having said that, we get the secondary problem that if rape takes place off-screen, it tends to be forgotten by fans of the rapist.

But rape should sometimes be committed by popular characters - the only convinced rapist I ever knew was a very charismatic man. We need to remember that rapists look and sound just like everyone else - that you can't spot them easily. (and the follow-on from that is that we have to have follow up to know what happens to both victim and rapist afterwards)