Was a pharmacist justified in shooting an unarmed robber 6 times?

Here are the facts: an Oklahoma City pharmacy was recently held up by two black teens. One teen held a gun on pharmacy employees while the other teen attempted to walk behind the counter presumably to steal drugs.

The pharmacist, Jerome Ersley who underwent spinal surgery 6 weeks ago, grabbed a loaded firearm from behind the counter and opened fire hitting the unarmed robber once in the head.

Ersley then chased the armed thug out the door and down the sidewalk, firing twice but missing both times. Ersley returned to the pharmacy where video surveillance footage shows him either reloading his weapon or retrieving another gun before standing over the wounded robber and shooting him five more times, killing him instantly.

Ersley is currently out on $100,000 bond. He appeared on The O’Reilly Factor last night to say he finished off the unarmed teen because he (Ersley) was disabled and he was afraid the boy would get up off the floor and kill him. Ersley said the would-be thief tried to speak to him as he pumped five more bullets into his body.

Question: Was Ersley justified in killing the thug?