If "law enforcement" on the border actually means law-breaking in practice, then it is no wonder so many illegally cross the border.
If the guardians of the law don't respect it, why would foreign nationals?
You are asking the people on the border to fight on unequal terms. Drug runners (many of whom have military training) do not follow rules of engagement, of any kind. This was not an ambush or sniping of someone who may or may not have been breaking law, this was after a physical altercation where one agent lay wounded. Yet you ask this man to retreat and call the agency legal team before reacting?
Try a ride-along with your local LE sometime to better understand how the real world works. It ain’t pretty.