Texas Penal Code - PENAL § 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.