His best bet for a plea is to second degree murder. No way any DA would dare to reduce the charge to anything that didn't include the word "MURDER".
This is a slam dunk murder case. The only bargaining chip the cop has is to plead to second degree and hope that someday someone will allow him to sneak out of the prison in 15 or 20 years on parole.
This case is a prosecutor's dream case. They have the cop pulling out his gun rather than chasing the perp (who was wanted only on child support warrants) and firing 8 rounds into his back and then trying to plant evidence on him after the fact.
It doesn't get any better for a DA than this one.
He's going to spend at least 20 years in prison and only if he pleads. Otherwise he's going to get the needle.
DAs hate to prosecute their LE buddies. He'll over charge and do a terrible job. Like the idiot in New Orleans who got his Danziger Bridge convictions thrown out.
I'd be surprised if this guy gets much more than time served.