This is primarily because rape has never been near as justifiable in human society as murder. I can think of a million and one quasi-legitimate justifications for murder. Furthermore, murder results in death, which has been a significant aspect of any human society for at long as this species exists. Whether from storms, or pestilence, or warfare, or by some other species, death has always been apart of our society. It is therefore not as shocking to see or contemplate a person die by the hand of another person, since the only difference is that person is being killed by another person rather than, say, the cold, or the heat, or bubonic plague. Rape is different. You do not get raped by tigers or wolves, by fire or water, by the heat or the cold, or by pestilence. Rape only ever occurs as a result of one person's action upon another. It leaves scars, both physical and emotional, that the victim will have to live with for the rest of their life, scars that cannot simply be written off as misfortune. Even theft or burglary can be said to occur "naturally", as I'm sure you will agree when rabbits raid a farmer's crops or a bear breaks in to your tent or cabin and takes what it will before you return to find it in ruins. Rape does not occur as such. Perhaps it is because of this that we can more easily witness or even understand murder, since death is a more common, unavoidable even, aspect of human society. Rape, to put it plainly, is not.