There's lots of things to assess in a scenario like this.
I know my limitations. I'm no resemblance to 6'4" 250 pound John Wayne nor any of the characters he played. I'm simply a 50ish, paunchy, out of shape desk jockey with some moderately good shooting skills.
That said, if a woman was being brutally attacked/raped/etc as the original post indicated, I'd have to do something. At night, I'd first hit the BG in the eyes with the light from a Sure-Fire Xenon light and yell "Get on the ground!" over and over, hopeful he will think it's a police officer and comply or flee.
In daylight or a well lit area, my hand would sweep the cover aside, take a firm grip on the sidearm and I'd yell the same thing, while moving sideways to him (harder shot if he's armed). If he complies, command the woman to move away (if she can) and get him into a prone position, feet crossed, arms out flat, palms up.
If attempting to use a command voice and posture fail and he continues to "brutally" assault his victim I'd have to end his actions with the most reasonable force I could muster.
Obviously if he is using a knife, tire iron or similar weapon and refuses to comply the decision making process is simplified over him simply assaulting the woman.
Once some semblance of control is attained, I'd probably call 911.
BillCA in CA (Unfortunately)