Sec. 9.42. DEADLY FORCE TO PROTECT PROPERTY. A person is justified in using deadly force against another to protect land or tangible, movable property:
(1) if he would be justified in using force against the other under Section 9.41; and
(2) when and to the degree he reasonably believes the deadly force is immediately necessary:
(A) to prevent the other's imminent commission of arson, burglary, robbery, aggravated robbery, theft during the nighttime, or criminal mischief during the nighttime; or
(B) to prevent the other who is fleeing immediately after committing burglary, robbery, aggravated robbery, or theft during the nighttime from escaping with the property; and
(3) he reasonably believes that:
(A) the land or property cannot be protected or recovered by any other means; or
(B) the use of force other than deadly force to protect or recover the land or property would expose the actor or another to a substantial risk of death or serious bodily injury.
yeah, it was not theft during the night time.....
HOWEVER IT COULD BE ARUGUED THAT HE WAS COMMITTING A BURGLARY (T.P.C 30.02) as he, without the effective consent of the owner, entered a portion of a building not then open to the public, with intent to commit a felony, theft, or an assault.
Behind the counter, employees only, stock rooms, etc., are portions of a building not then open to the public and would constitute a burglary
Note that could bring into play this section of the Penal Code
Sec. 9.05. RECKLESS INJURY OF INNOCENT THIRD PERSON. Even though an actor is justified under this chapter in threatening or using force or deadly force against another, if in doing so he also recklessly injures or kills an innocent third person, the justification afforded by this chapter is unavailable in a prosecution for the reckless injury or killing of the innocent third person.