Yes, I agree, I would have expected him to arrest her earlier considering his treatment of her.
There must have been some harsh words spoken between the two.
The only thing I can think of as to why he didn’t arrest her is that he didn’t have cuffs or flex on him, but that couldn’t possibly be the case.
Maybe he didn't arrest her or cuff her because he wasn't a cop. Or maybe he was off duty because he isn't dressed like a cop.
MAYBE (and this is what I think happened) he offered to give the young lady a ride home and she refused.
Then he told her he would have to search her for drugs (threatening her with arrest if she didn't just go with him).
He chased her down, pinned her on the bench, and searched her purse. Then he let her up. He probably offered again to give her a ride, and she refused and tried to escape. He catches her, then after a few seconds, slams her to the ground again.
The other on duty cops come out, and he tells them she attacked him. She gets up and tries to tell them what happened (and what a creep he is) and he says she is lying.
That's when she charges him with the knife.
She gets shot, no audio for us to hear, and no one is the wiser (at least of the general public).