One: The video appears to show that the perp was veering away from the cop when the first shot was fired. Maybe that's true, maybe it's not. That's for a jury to decide.
Two: The video appears to show that the cop continued to fire after the perp had fallen to the ground. It can be argued that the perp was still a danger to the cop. But again, that's for a jury to decide.
Three: The DA claims that the perp's knife was folded into the handle when he was shot. If that's true, a jury will convict in less than an hour.