Though not the recommended course of action... Texas does have a self-defense statute that allows you to use comparable force against another when your person or property are threatened.  There is not a specific formula that the law prescribes-- but instead uses a "reasonable person" type standard.  For example, if a person is threatening you with a gun during the robbery-- you can defend yourself and your property with a gun.  However, if the person just says "give me your money" and does not have a weapon and does not purport to have a weapon, the choice to use deadly force may not be considered reasonable-- because it's not comparable to the amount of force being exerted against you.  You could still raise it as a defense, but it will be up to a judge or jury to decide if your use of force was reasonable under less threatening circumstances.