He failed when he chose to pick up a gun and do what he thought was needed to make some money.
He also failed to realize that an old white man was just waiting for the chance and moment to kill a black man. He failed to realize that once he was shot in the head, there is a chance that this racist would take that moment to kill him.
He put himself in this situation. And the law has done more than enough to discourage people from going past the point of defending themselves and being judge, jury and executioner.
He also made it an example that the next time some blood thirsty gun owner is out to kill someone in "self defense", they need to make sure no one is watching, it isn't recorded and they kill them in one to two shots. Clearly, those who bring up this point do not recognize the law and wish to break it.