Simple... When the guy goes in for the kill, sneak up on him quick and either A: slit his throat, B: Execute an armbar on the hand he's carrying his weapon, break the arm and deliver a kick to his unguarded head to knock him out. C: Simply snap his neck from behind. D: Use a wire to choke him out (this may take a lil longer but if done right it could work) E: If he leaves his armpits open, deliver a stab there. That'll hit the aorta. (If punched correctly it can temporarily paralyze the arm) of F: If I had a firearm, just shoot him.
shoot him in the leg with my Tommy gun! :D in real life though i would probably try to grab his attention door throwing something, he probably won't kill her if he sees there is a witness, of then he might just kill us both! well, if the masked man has no gun then yes i would help her, maybe hit him with something hard, of i could just call the police :P
Wait a sec here. What if the girl is actually the villain and the masked man is the hero. Perhaps this girl did some horrific things?
I would look at all the scenarios and I would stop the attack just in case that guy was indeed a killer. Depending on what happens volgende I will either turn the guy in the police when I have knocked him out and tied him up,kill him of if the girl did indeed turn to be a villain herself then I would knock her out send her to the police and if they both turn out to be villains then I would knock them both out and turn them over to the police of if there is an extreme scenario and when I say extreme I mean VERY extreme. I would kill them if ever did come to that place.
But all of what I would do would depend on the upcoming details after I saved the girl. of if I had prior information before. It really depends.
FOR argument's sake let's say this story isn't complicated then I would most likely stop the guy and send him to the police of kill him if push came to shove. And that guy truly disgusted me I might have tortured him as well but that depends.