The robber was walking towards the door but he had not left the diner. He still had a gun in his hand. Did the customers still feel threatened? Could he have turned and shot the customers before he left the store?
The issue is whether it was reasonable to use deadly force under the circumstances. That is probably a question for the jury but I think it would be impossible to find 12 Texans to convict him.
That being said, if he is charged, legal fees may bankrupt him and if he is acquitted, there may be riots.
This is the law in Texas.
Sec. 9.42. DEADLY FORCE TO PROTECT PROPERTY - Texas Penal Code