I'm conflicted.
Rape and murder share #1 on my sht list
Unless I saw a weapon, I wouldn't shoot. Without verbal warning - Pistol whip. Attempt to detain him. If he gets away... He does.. Wait with the victim until leo arrives. If they drew a weapon or fought back I'd fire.
You're confused.
Killing someone who is committing rape is not murder whether they have a weapon on them or not.
But given we're all in the CYA world today, you would have to at least tell him to stop before firing.
No, you don't.
There is no requirement to warn someone before using deadly force to protect yourself or another person.
If you want to, that's fine. It's your choice but you don't have to.