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.
Texas Penal Code, Chapter 9, subsection 41 and 42 - also known as 9.41 and 9.42.
§ 9.41. PROTECTION OF ONE'S OWN PROPERTY. (a) A person
in lawful possession of land or tangible, movable property is
justified in using force against another when and to the degree the
actor reasonably believes the force is immediately necessary to
prevent or terminate the other's trespass on the land or unlawful
interference with the property.
(b) A person unlawfully dispossessed of land or tangible,
movable property by another is justified in using force against the
other when and to the degree the actor reasonably believes the force
is immediately necessary to reenter the land or recover the
property if the actor uses the force immediately or in fresh pursuit
after the dispossession and:
(1) the actor reasonably believes the other had no
claim of right when he dispossessed the actor; or
(2) the other accomplished the dispossession by using
force, threat, or fraud against the actor.
They lose on 3. There's no way for WM employees to reasonably believe that.
Given that the guy was already in custody and immobilized, there were no longer any fears about recovery of lost items, nor any danger of harm from the perpetrator.
Given that it took 20+ minutes for this guy to die in custody, and given the responses of the guards, negligent homicide charges would seem to be justified here.