Tennessee v. Garner look it up.

There are some justifiable circumstances for the use of deadly force. Shooting a fleeing unarmed suspect, in the back, is not one of them. Notwithstanding some special circumstance of the suspect's intent of lethal danger to other police officers or the immediate civilian persons.

This looks like a bad shoot to me if the witness accounts are accurate.

Coulda, woulda, shoulda should make no difference.