Did he shoot the previous robbers in the head as well, but they ended up getting away?
Because the only thing that would justify him shooting an already downed robber, according to your logic, is that the past robberies resulted in him shooting them in the head,... and they still get away. So, this time, he had to make sure they were dead.
I agree with defending yourself. I don't agree with taking it into your own hands to dish out punishment, well after the fact you stopped a criminal and defended yourself. There was no reason to shoot the criminal, who was already shot in the head, 5 more times.
It was emotional, but it was hate. He knew what he was doing. He let his hate get the best of him and he showed he was just looking to end a young black man's life.