Let's assume WORST case. Instead of a security guard, let's assume that this was an actual LEO doing the tackle.
What does TX law say about LEO's using force?
§ 9.51. ARREST AND SEARCH. (a) A peace officer, or a person acting in a peace officer's presence and at his direction, is justified in using force against another when and to the degree the actor reasonably believes the force is immediately necessary to make or assist in making an arrest or search, or to prevent or assist in preventing escape after arrest, if:
(1) the actor reasonably believes the arrest or search is lawful or, if the arrest or search is made under a warrant, he reasonably believes the warrant is valid; and
(2) before using force, the actor manifests his purpose to arrest or search and identifies himself as a peace officer or as one acting at a peace officer's direction, unless he reasonably believes his purpose and identity are already known by or cannot reasonably be made known to the person to be arrested.
It would seem that even an LEO is required to announce his purpose to arrest and search before using force... A blindside tackle without giving the tacklee previous notice would even put a TX LEO in legal jeopardy.
BUT, let's say that a TX LEO DID give notice of arrest and then tackled a citizen, but in the absence of resistance or attempted flight. What would a citizen be justified in doing in those circumstances?
§ 9.31. SELF-DEFENSE. ...
(c) The use of force to resist an arrest or search is justified:
(1) if, before the actor offers any resistance, the peace officer (or person acting at his direction) uses or attempts to use greater force than necessary to make the arrest or search; and
(2) when and to the degree the actor reasonably believes the force is immediately necessary to protect himself against the peace officer's (or other person's) use or attempted use of greater force than necessary.
(d) The use of deadly force is not justified under this subchapter except as provided in Sections 9.32, 9.33, and 9.34.
So, even if this HAD been an LEO making the tackle, the citizen would have been justified in using force to resist given that he had not offered any resistance before the tackle.
But, is drawing force? Or is it deadly force?
§ 9.04. THREATS AS JUSTIFIABLE FORCE. The threat of force is justified when the use of force is justified by this chapter. For purposes of this section, a threat to cause death or serious bodily injury by the production of a weapon or otherwise, as long as the actor's purpose is limited to creating an apprehension that he will use deadly force if necessary, does not constitute the use of deadly force.
It would seem that merely drawing his pistol is justified if force (not even deadly force) is justified.
So, by the letter of the law,
even if the tackler had been an LEO who announced his intent to arrest, New Master's actions are perfectly justified as long as he had not resisted or attempted to flee. Furthermore, the LEO would have acted in a manner that is clearly contrary to the law.
Given that it was NOT an LEO AND there was no notice prior to force being used AND there was no attempt to resist or flee prior to force being used, I would say that the person doing the tackling and his employers are almost certainly liable civilly, and had New Master known the law, he could probably have pressed charges against the tackler and had him arrested.
BUT, what if New Master really HAD been shoplifting--and this is the kicker. Nothing changes!
There is absolutely no justification for what was done to New Master. Even if he HAD broken the law by shoplifting, that would not have allowed a security guard or even an LEO to tackle him in the absence of resistance or attempted flight. And, even as a shoplifter, under such an onslaught he would still be entitled to defend himself with force as described by § 9.31.