The rules in Texas say that you can come to the defense of another person if that other person is in mortal danger, as in danger of losing their life.
Looks like this was the case here.
the robber was running.. Dude might have been giving chase. Again how does ANYONE know the guy chasing the robber is telling the truth?
Again I am glad they caught the bad guy, but, what if the guy chasing the other was really the bad guy and they shot the wrong person?