I am not sure why various folks here are hung up on this "running away" business
It's a hollywood idea that's been mainstreamed. I'm not saying it's right, or that it's wrong. I'm saying it's the way it is. We have some level of defense available to us, but it'll be judged on a case by case basis, and almost always less so than an actual officer who will often have technically more restrictive rules on such a shooting.
In my state, I could probably point to our state laws, and convince a lawyer and a judge. The Jury would probably be at best even money given state politics.
I could argue he was committing a felony in my presence, and on me. This is grounds for deadly force by both civilians and LEO in WA. I could argue that given his rapid response to go from watch it, to pulling a knife, as well as his repeated efforts to clash with me, he posed a significant threat of death or injury to myself and/or others.
I'm not a lawyer so I may have the nuance wrong, but that's pretty much the end result of Tennesee v. Garner 471 US 1 (1985) for when a law enforcement officer is able to justify deadly force on a fleeing suspect.
The last thought I'd leave you with is that it's still a crap shoot. You could get an activist judge, you're going to go broke from legal fees, and worst of all, you will have shot someone. With the public stigma of it being "in the back".