My reasons:
*Rape is showed less in all media productions, thus the viewer being less used to it, causing a larger outrage.
*Rape is usually (don't start calling me sexist- you know it's true) a man raping a woman. Most of the time. Now I'm not saying that the movie itself is sexist, but woman are (once again, overall, this is true) considered by society to be the weaker sex. It's been that way for generations. So, by a scene of a man raping a woman, it's not only awkward- but considered politically incorrect. So... it's kind of "worse" in that aspect. Because killing someone isn't as politically incorrect. Yes, I just said that. Why? Because, a man can kill a woman, a woman can kill a man, hell, in most movies today they'res psychopathic four year olds killing people. But how many times in movies do you see a woman raping a man? You don't. Now I'm not saying that taking someone's life is more morally stable than rape, in fact I am not saying that at all, but it's simply less "wrong". Note the quotations.
*I also think, as others have stated, sex scenes are much more realistic than murder scenes... (how many times do you hear irl of a saw act occuring?) But rape DOES happen. So, it's more... scary. Seriously. Not to mention sex itself is dramatic to watch in media.
I know I'm gonna get a million "SEXIST! YOU'RE CALLING MEN RAPISTS!" Responses. However, that's not what I'm trying to point out here