honestly....I cannot believe you are bitching about someone who fucking stopped a murderer BEFORE you are complaining about the motherfucker who killed a cop.  That to me just seems beyond ridiculous.
he didn't just walk up and shoot him.  He picked up the gun and asked the guy who just murdered a cop to drop his gun and stop.  They guy didn't, and was then shot.
seems perfectly fair to me.  Why let the murder who just killed someone right infront of you get away?  What if he went on to kill someone else?
Seriously....if I had a chance to kill that motherfucker I would have in a second.  He had already lead the cops on a high speed chance, putting a ton of other people at risk, and then shot and killed a cop.  So how can you  honestly say he was not be placed at risk?  If someone is around me shooting a gun, I would consider myself to be in danger.  His fucking kid was with him too.  I would protect my own, as I have a right to.
Would you opinion be any different if he had been carrying his own gun?  I am not a proponent of guns at all, but if I was in a situation where people were being shot and I could stop it, be it with my weapon or another.  I wouldn't just stand there and say "O, I am mad, but it isn't my problem."  I would fucking get the gun and fight back, even if it wasn't me who was directly being threatened.  It is no different that stepping in on a drunk fight at a bar or an old lady getting mugged, only this time it is for real and you might die if you fuck up.
The guy who stepped up, instead of being a bitch and letting a cop killer walk away, is a hero, no questions asked.  If you are more worried about him getting charged with attempting to make a difference than you need to reevaluate your preceptions about the world around you.