He RAN the guy down in his SUV after the robber left? Sorry, but the threat to him left when the robber ran away.
Manslaughter is a good charge here. He was not defending himself or his property at the point he drove down a 1 way street to get him.
Now, had defendant backed up while the robber had the gun in the his face and ran him over, that would be a different story.
The guy went to far. Now if he was afraid that the guy was gonna come back and potentially finish the job, (i.e. kill the witness) then he should go out, buy a 12 guage, wait for the bastard to get in the front door and then pop him 2 or 3 times. There isn't a jury in the world that would convict him of any wrong doing in that situation because, the IMMEDIATE threat was there (i.e. the robber in the house) AND because of the past history of the prior encounter.