In Virginia, use of force (up to and including deadly force) is legal if you fear for your own life, or for the life of a third party. That being said, I would not likely intervene unless I had witnessed the entire situation and had absolutely no doubt of what was going on. If I felt 100% certain of the facts of the situation, and was in a position to neutralize the BG with negligible risk to the victim or bystanders, I would probably skim the gene pool, but it would depend on how I felt at that moment. I have no desire to be a hero. If the female in the scenario were my wife, sister or mother (I don't have a daughter) I would most certainly shoot.