This is straight up murder. There us no defense.
It could be negligent homicide. He may have been reckless in unholstering his weapon and pointing it at the driver's side door "just in case" and he accidentally pulled the trigger. She was hit in the stomach so I would really like to know whether the round went through the car door.
The County Corner has already ruled her death a homicide.
Only other witness is the other cop.
He will walk
He feared for his life. Shot her, and shut up. This officer will not be prosecuted for murder. Possibly a lessor degree of manslaughter at the most.