Let me put it this way, if someone is willing enough to try to force their member inside of me, I will damn sure know that they have no positive intentions for the course of the next 5-60 minutes. Included in those intentions may quite possibly exist the intent to kill me to conceal their identity.
Additionally, given that the person is willing to try and do something like that to me or someone else, I would be willing to bet that they'd be willing to try it on someone else in the future. Thus I can conclude that they have little regard for the well being of others. That said, I'd be willing to bet that they would try their actions again on someone else.
So, rather than call it preemptively killing when not knowing what will happen next, I will base my actions on what I know is about to happen. If that seems to be rape, if I believe my life or body is in danger, I will kill the attacker. No questions about it. I will do anything and everything I can to stop the attack. If he dies, that's his problem... not mine.
And, whether I use a gun or not to defend myself is entirely up to the tools I have available to me at that time to stop the attack.