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Texas Penal Code 9.33 and 9.34 spells this out pretty well:

Sec. 9.33. DEFENSE OF THIRD PERSON. A person is justified in using force or deadly force against another to protect a third person if:

(1) under the circumstances as the actor reasonably believes them to be, the actor would be justified under Section 9.31 or 9.32 in using force or deadly force to protect himself against the unlawful force or unlawful deadly force he reasonably believes to be threatening the third person he seeks to protect; and

(2) the actor reasonably believes that his intervention is immediately necessary to protect the third person.

Acts 1973, 63rd Leg., p. 883, ch. 399, Sec. 1, eff. Jan. 1, 1974. Amended by Acts 1993, 73rd Leg., ch. 900, Sec. 1.01, eff. Sept. 1, 1994.


Sec. 9.34. PROTECTION OF LIFE OR HEALTH. (a) A person is justified in using force, but not deadly force, against another when and to the degree he reasonably believes the force is immediately necessary to prevent the other from committing suicide or inflicting serious bodily injury to himself.

(b) A person is justified in using both force and deadly force against another when and to the degree he reasonably believes the force or deadly force is immediately necessary to preserve the other's life in an emergency.

Acts 1973, 63rd Leg., p. 883, ch. 399, Sec. 1, eff. Jan. 1, 1974. Amended by Acts 1993, 73rd Leg., ch. 900, Sec. 1.01, eff. Sept. 1, 1994.