Texas Penal Code section 9.04:
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.
That means that if you’re in reasonable fear that someone will cause you bodily injury (bodily injury simply means pain) you can threaten to cause serious bodily injury or death.
So, for example, if you’re charged with committing aggravated assault by threatening a person with a firearm, to establish a self-defense claim (or a defense-of-others claim) you don’t have to show that you reasonably feared that they would kill you otherwise, but only that you reasonably feared that they would hurt you otherwise.
For some reason I hadn’t happened upon section 9.04 in a dozen years of practicing law. But it turned up when I needed it.