Nice hypothetical. Defense of persons takes precedent over defense of property, and neither applies when the homeowner is engaged in a crime against someone who was formerly allowed on the property.
Technically, one should then leave the property and call the cops.
Now obviously my hypothetical is an extreme case. But as far as I know maybe the "meathead" was irrationally angry because their child was being abused by the landowner and the child was in the property. Or maybe none of that is true and he was angry because he is a belligerent jerk. Still seems flakey to me to call this shooting self defense...a crime against language if not against Texas law.