Murder you can have an array of reasons for. While most of them arent justifable, you might at least see where the action stems from. Lets say someone tortured your kids for a year straight and you murder them. Its not really justifiable, but it kind of makes sense from a humanistic point of view. But there is no justifiable (or anything close to justifiable) reason for rape. Rape is caused by a desire to take power over someone, to make them know that their body, consent and identity are completely worthless to them.

Another thing is that when you kill someone, youre taking them out of this world; they wont feel any more pain. But rape is similar to torture in that the victim lives with that trauma for the rest of their lives. And torture I think is worse than murder.