Legally, it's complicated, but his actions caused two people to die. Nobody else killed anybody there. From my view, he was looking for trouble and he found it. This incident is the reason I think we need more gun control. He was a kid with a gun who made a bad decision getting involved in something he couldn't handle. It seems he is the type of person who shouldn't be carrying a gun.
The vigilante act was him going into a powder keg with a gun. I realize he didn't necessarily go there to kill anyone, but his taking the law in his own hands caused two people to die.