In Morales v. State, the Texas Court of Criminal Appeals held that there is NO duty to retreat when you are rightfully defending another person. The CCA held that:
A person is justified in using deadly force against another if he could be justified in using force against the other in the first place … and when he reasonably believes that such deadly force is immediately necessary to protect himself against the other person’s use or attempted use of unlawful deadly force and if a person in the defendant’s situation would not have had a duty to retreat…
Therefore a person may act against another in defense of a third person, provided he acted upon a reasonable apprehension of danger to such third person, as it appeared to him from his standpoint at the time, and that he reasonably believed such deadly force by his intervention on behalf of such third person was immediately necessary to protect such person from another’s use or attempted use of unlawful deadly force, and provided it reasonably appear to such person, as seen from his viewpoint alone, that a person in the situation of the person being defended would not have had a duty to retreat.
A person who has a right to be present at the location where the force is used, who has not provoked the person against whom the force is used, and who is not engaged in criminal activity at the time the force is used is not required to retreat before using force as described herein.