How do we know he wasn't leaving the room to go get the kids and carve them up to make the guy talk?
We don't.
And given the behavior her been engaged in, prior to his being shot, I think quite likely he still posed an immediate threat, regardless of whether he had his back turned.
I'm not trying to argue that the shooting of him wasn't justified, I expect it was. I'm only complaining about some of the reasons that are being proffered on this board as reasons for his being shot.
You or I may think that he deserved to be shot, but deserved or not deserved are judicial questions, and have no relevance to self defense.
The only question at issue in self defense is "did he pose an immediate threat?"
If you're ever involved in an incident of self defense, do not talk about what he did or did not deserve. It'll get you in trouble.