Look, I can't even believe this is such a controversy. A 15 year old is not a child, a 15 year old knows not to throw rocks at people. A police officer (or border patrol agent) is issued a firearm and trained to use it. They are also responsible for that firearm. This means that if someone, or a group of people, are attacking you in a way that could knock you unconscious, resulting in potential loss of control of said firearm, you do you what you have to do to prevent that scenario.
Like someone else said already, time does not go slow-mo like it does in the matrix when you pull your gun out, I doubt he intended to hit the boy in the head but in the heat of the moment you can't really rely on shooting for appendages, he likely shot for center mass as was already suggested.
For the nonsense about what if this had been an American in a US suburb... that is called dropping context. The fact is this was an illegal alien who has priors for smuggling across the border, when border patrol agents attempted to arrest people he was with he and others in the group started throwing rocks at the agents.
Even if it were a group of American teenagers who started throwing rocks at police officers in a suburban neighborhood while the officers were trying to make an arrest I would feel EXACTLY the same way as I do about this case.
The officers have the responsibility of retaining control of their issued firearm, if they are knocked unconscious from a rock to the head (which can EASILY happen so knock of the retarded nonsense) then they lose control of that firearm and put INNOCENT people in danger.
Smugglers who are in the US illegally and who are assaulting officers are NOT innocent people, stop treating them as if they were.
Since when do we feel bad for people who are doing wrong? Don't cross the border illegally, don't smuggle while crossing the border illegally, and don't throw rocks at border agents while smuggling and crossing the border illegally and you won't be shot at. If it were up to me we would have guard towers ever 50 meters and shoot on sight, laws are meaningless if not enforced, and the wave of illegals sweeping into this country every year tells me we are not adequately enforcing the laws.