And the jury agreed with you. And after looking at the footage I agreed with them. In the UK though, showing up anywhere as a vigilante armed and dangerous then killiing someonew with your weapon would see you going to prison for manslaughter. Showing up with the weapon is what a British jury would've likely focussed on.
The American legal system and the laws it has have worked well in this instance - the issue is whether those laws are the laws that the country should have. That's the discussion that's being missed and it's one that will require cross party agreement - but I suspect that won't happen. American politics is a game of under informed polarisation and this comments section should demonstrate that easily. This article is being attacked by both the right and the left.
Ultimately, a young man (still legally a child) was armed and policing a situation that shouldn't have been allowed to escalate. He was under prepared, under trained and out of his depth pretty quickly. Had this been a civil war, he would've been applauded for his actions... but it wasn't a civil war, it just looked like one.