It depends. If, as I said, there were no testicles present (metaphorically speaking), then what weapons might have been handy means nothing.
If I'd been there, unarmed, I can say with assurance I would have grabbed whatever was handy and attacked the nutcase.
However, a lot of people don't have the confidence I have to engage in hand-to-hand with a guy with a knife; having a pistol and shooting skills could make a big difference in whether they'd intervene. If some law abiding permit holder had been present and armed, the guy might have been shot dead before he inflicted fatal injury on the girl.
I don't see how you can say you're 99.99% sure the girl would have died anyway. You'd have to be assuming that no one present would be armed (about 1 person in 50 carries a gun), and/or that the shooter would not respond or not respond effectively (this depends on the person, their character and training, and such speculation is difficult).
Even if we assume a 50/50 chance (if carry permits were allowed there) that someone present was armed, and a 50/50 chance that such a person would respond effectively, then that would have given the girl a 25% chance... which was better than the effectively-zero chance she got.