Yes, I would convict. There was no further threat after the accident. And, no, I won't buy the "well, if the guy who hit them drives again...." garbage. There was no immediate threat so no justification of use of deadly force.
He had to walk back to his house to get the gun. That, to me, is the critical point. If he had pulled his gun at the scene, I would reluctantly say that the trauma of the crash had him dazed and without impulse control, and be willing to call it voluntary manslaughter, sentenced to time served. Possibly even involuntary manslaughter.
I could still, with sufficient medical evidence, be convinced that due to trauma suffered in the crash he had no control of his actions, but it would be a tough job to convince me. Murder 1? No. Murder 2? Most likely no.