What she was doing in the video was well outside the bounds of "self defense" unless you count "the best defense is a good offense". The woman she was trying to stab was facing the police officer who was saying "Hey, what's going on" at the time. Self defense requires an imminent threat.

The officer also said "get down" four times (after shouting "hey" in increasingly agitated tones several times) before firing--I'm not sure what you mean about "prior warning".

We really need more work on nonlethal interventions so we're not expecting police to make split-second decisions about their only two options: "use words" and "death".