He was killed during an assault of a border patrol agent. Plain and simple. While it was incredibly unfortunate it happened, it still doesn't diminish the fact that the border patrol agent was there doing his job, trying to detain the two assailants, and it can be seen that shit was hitting the ground around him, even from that shitty quality cellphone video.
He didn't ride up on the kid with a rifle, sniping him as you alluded earlier in the thread. He didn't slink along the top of the bridge, viciously executing the kid and shooting him in the head from the high ground as you've tried to point out plenty of times.
He simply feared for his safety, with a shit load of idiots running around him, throwing shit at him, and he took steps to ensure his safety.
Fact is, these people know damn well better than this. The fault in this incident is on their own heads and the head of the kid killed. The border patrol officer will feel incredible guilt about this, guaranteed, they never go into a situation hoping to kill someone, but those idiots that helped create the atmosphere for this confrontation will have to deal with the fact that they killed their friend.