That's a tough break, freak accident.
I can see where the officer see's stranded motorist with weapon, he feels threatened.
I can see where stranded motorist feels threatened by non-uniformed guy stopping.
If stranded motorist would not have had a gun, it probably wouldn't have happened.
If off duty policeman would not have had a gun, it probably wouldn't have happened.
If neither had a gun, it absolutely would not have happened.
Sad deal.