Prudent man rule would be easy in court:
Prosecutor: "Why, Mr. Plumber did you chase down the alleged shooter and kill him as he fled?! He was obviously not a threat anymore since he left".
Mr. Plumber: "I killed him because I thought that if I didn't he would be an imminent danger to society. Case in point, he just shot up a church. I thought that if he was going to do that, maybe he's on the way to shoot up YOUR church Mr. Prosecutor".
Judge/Jury: "Not Guilty. Case dismissed. Prosecutor, will you please step back into my chambers? "
So once Plumber jumps in the car was Plumber trying to murder him, apprehend him, or just follow him so that cops would know where he was going? Kidnap victim, if there is one, changes the scenario.
If it was that SOB is fleeing ..I am going to murder that SOB for what he just did...there is a problem.
The best scenario for Plumber is shootout at church and Psycho killer dies from that gun fight at Church or self inflicted
If Plumber executed him after Crash he could be looking at a Civil case. Yes, it sounds crazy but it could happen. Might be hard to win but it could happen.
Once he is fleeing at 100 miles an hour to get away from Plumber and any cops coming..Psycho Killer might have been using self defense because Plumber could then be seen as the aggressor and he is not law enforcement