that's a tricky one. I know by the law, should NOT fire unless he turned back around with a weapon.
WRONG! WRONG! WRONG!
The requirements to use lethal force in most if not all states are:
Your attacker has the means to inflict death or severe bodily harm
Your attacker has the intent to inflict death or severe bodily harm
Your attacker has the opportunity to inflict death or severe bodily harm
All three conditions were met.
The fact that the man's attacker had his back turned is completely irrelevant.
The attacker retained opportunity by remaining in the near vicinity.
He had exhibited intent with his initial violent assualt and was exhibiting continued intent by not retreating and reaching under the seat to retrieve what a reasonable man would assume to be a weapon.
He had the means as demonstrated by his initial violent attack.
Shooting the attacker in the back would be completely justified under the conditions described.
Even in states where the law mandates retreat that mandate is eliminated if retreat is not possible. The man's wife was on the ground, the man was on the ground. Retreat wasn't an option.
Shooting the attacker was the only option.
Like the Sheriff's Deputy remarked: Too bad the man missed.