The BP agents stated that he was unarmed. He was running away from them, they knew he was unarmed, and they shot at him anyway. That's criminal behavior, and they are being punished by prison time.
If they did discharge their weapon inappropriately, that is a matter for the Border Patrol to handle internally. The men should have been fined, fired, demoted, whatever according to agency disciplinary rules. It is handled that way in every branch of government from the police force to the military.
Also, they were found guilty under a statute that does not apply: the use of a gun during the commission of a crime. But they weren't committing a crime. They were doing their duty protecting the border. So that statute didn't apply to them.
This Johnny Sutton is another Niphong.