Maybe they were unclear about what actually happened. The son of bitch deserved to be shot, but once he was neutralized no sane person would go grab another gun and shoot a defenseless person, even if he were a shithead thief.
My little brother used a golf club to beat the tar out of a (very large) asshole he caught breaking into his car in our mother's driveway. I was proud of him for that. I wouldn't have been proud of him had he continued beating on the guy once he was down and not getting up. Waiting for the police to arrive was the right move (and the cops agreed- the very large methhead thief lived to apologize in court before being sent up the river for a spell).
Heat of the moment counts for something so the sentence is arguably heavy, but firing off shots in public at the fleeing robber and executing the unconscious one- those are crimes.