Watch,watch,watch. Yes, he may have had a reason to WATCH him. He had no authority. (Remember he was instructed by police to stand down.) As soon as 911 says,"STOP!." He should have stopped. But no, he didn't. He had no right to follow, confront and kill TM. Period.
And civilians don't get probable cause, because they are not legally authorized investigators. So he really should have stayed in the truck, which is why every one is pissed. And if TM was your kid, you'd be outraged at the blatancy of this situation.