It’s not as clear cut as that. Until 1985 a cop could legally kill any fleeing felon. SCOTUS narrowed the law via the Garner case but still allowed a cop to kill the perp if he considered him to be a serious threat to himself or others.
This is where it gets a little grey. Just moments before Slager shot Scott, Scott was fighting with him and had grabbed his taser. This made Scott a mortal threat to Slager. It was then that Slager justifiably decided to draw his gun and kill him. Slager’s mistake was in failing to unmake that decision once the situation changed and Scott started to run. But even then, you’re talking about someone who was willing to fight a cop and grab his weapon. What if Scott had run to the nearby pawnshop and beaten someone up?
You say that’s far fetched but consider the situation from Slager’s POV given what he had just been through with Scott. That’s why I think the murder charge won’t stick.
Officer Slager picked the fight. His mistake - a bad one.
Eight shots at a fleeing man and a filed report that is contradicted by video evidence - evidence that includes dropping the taser by the body (planting evidence) doesn’t leave a lot of grey area. There was no need to kill that man in that situation.
A man facing a murder charge who has conclusive evidence against him also indicating obstruction of justice (dropping the taser by the body after the kill) and falsifying official documents is not likely to have any other part of his testimony believed at trial.
My guess is that he’s looking at a 20 year sentence.