Guy is walking down the street across from some apartments. A guy who lives in the apartments starts threatening the walker from his steps of the apartment. The walker keeps walking. The guy from the apartment keeps yelling "I'm gonna kill you, you privacy rapist" (lot of whackos out there) and then charges the guy with a knife. Ultimately in real life the walker was able to escape and the attacker went back home. The cops showed up and the walker ID'd the guy and the knife which the police found in his apartment.
SO obviously a fight or flight scenario. But if someone charges you with a knife (you are alone) and you have a gun do you run away and hope you are faster or pull the gun and put him down?
And by the way, it was clear from the post arrest comments that the attacker wasn't all there. Does that make a difference?