Telika: So, question, why is rape worse than murder in that respect ?
In my experience, the reason is that ...
a) rape (in all its forms) affects a much higher number of people, so you're more likely to encounter people who will feel bad when you talk about it in a casual manner
b) Murder or at least deathly violence _is_ a common aspect of gaming, while rape most certainly is not. As a result, coming from a battle in an MMO and saying "I totally killed that guy" is an appropriate description of the event, while saying "I totally raped that guy" introduces a connotation of sexual violence that has nothing to do with the actual event.
Objectively (if we assume that these things can be rated objectively, which I doubt, since they are determined by culture) rape might not be worse than murder, but in my experience, trivializing rape in casual conversation clearly makes more people feel awkward than trivializing murder.