She was a young woman likely in search of a better life who was crossing illegally, but while she can’t break the law, he also can’t shoot her. If she was attacking him, he could, but if he just shot her, that would be wrong. We don’t support shooting unarmed people. He will, if he did this without cause, be charged with her death.
This is a reason why I think we need a barrier at the border because not only is our current system not working, a barrier would decrease physical confrontations and danger for both sides - particularly if the wall had see through capability. Our agents are putting their bodies on the line, but as we can see, an illegal immigrant also got killed.