Dannyalcatraz said:"Compelled" in this context means driven to act by the acts of another. That is, he didn't use deadly force just for the hell of it, nor did he intend to kill that guy as soon as he found him, or any other hunting scenario, but instead had a reasonable justification for doing so- namely saving the life of another.
Interesting. he doesn't have to be compelled to use lethal force to be justified in using it? He only has to be compelled to act in some way, and then he is justified in uusing lethal force?
His options- do nothing (law doesn't require a civilian to act except under certain circumstances not met here); go for help (which may not prevent an assault which could end in murder, and the assailant might escape in the delay between departure and return); call out (which may have stopped the assault or resulted in a fight he doesn't know if he can win, after which the assault will resume); or immediate use of deadly force.
I note that you implicitly discount the possibility of immediately using non-lethal force. Why?
"Necessary" in this context would mean that the person using deadly force believes that there is no other safe way to prevent the action being attempted.
Does that belief have to be reasonable? I would have thought that a reasonable paladin (there's a notion for AP Herbert) in that situation would have believed that there was another safe way to prevent the rape or other assault.
The law does not require a civilian to put himself in harms way to prevent an injury before resorting to deadly force.
law enforcemment officers are held to a higher standard, I take it?
Here, we don't know what would have happened if the Paladin had shouted "Hold, miscreant!" Given the locale, he might have just been cruising for a bruising- who knows if the assailant had allies in the bar?
I consider that just as speculative as the possibility that the rapist might have been acting under a compulsion or possession. Basing our judgement on chimeras is an axe that cuts both ways. Imaginary allies, imaginary knives, imaginary demons, imaginary spells, imaginary fears: they aren't the things that a reasonable person bases his or her actions on.