dpg, you asked--"That the prominent man killed the guy is never in question ? is it? So the dramatic question is WHY? Was it justifiable manslaughter? Or??"
The dramatic question may be more HOW than WHY. You're right, the fact that Jim kills Billy is not in dispute. Jim claims Billy fired at him first and he fired back in self defense.
The State's Position: There is no gunshot residue on Billy's hands. They believe Jim murdered Billy and fired Billy's gun himself to make it look like self defense.
Defense Team Position: Shoddy police work. The investigation has been botched from the start as evidenced by Jim's cat being allowed to walk around the crime scene (a true story). The gunshot reside must have gotten wiped off Billy's hands in transit to the morgue.
This ramps up the stakes for Jim--he will either be charged with first degree murder or set free.