[snacktrack]

Obviously if he is using a knife, tire iron or similar weapon and refuses to comply the decision making process is simplified

I would say so, as simple as a double tap.

In this situation I would go with my gut, and wouldnt think the hell out of the situation. Seconds would be critical in a situation like this. I wouldnt call 911 first, I would focus on stopping the attack. If he was using a weapon I would fire. If he was using his hands, I would yell loudly for him to stop or I will fire. I would call 911 immediately if the BG flees, or I have gotten the woman to safety.

This is a park, and assuming it is secluded I wouldnt give the BG the benefit of the doubt of non lethal intentions. As someone else said this may be the woman's last few seconds of life. I would rather save hers, a life cannot be taken back. Charges can be dropped, trials can be appealed, there are many options legally. Her family will thank me that I did not think of myself.