§ 9.32. DEADLY FORCE IN DEFENSE OF PERSON. (a) A person is justified in using deadly force against another:
(3) when and to the degree he reasonably believes the deadly force is immediately necessary:
(B) to prevent the other's imminent commission of aggravated kidnapping, murder, sexual assault, aggravated sexual assault, robbery, or aggravated robbery.
In other words, the State couldn't have prosecuted the father if they wanted to. In this case, as I understand it, a no-bill by a grand jury is usually a formality to ensure that jeopardy has attached so that the individual cannot be prosecuted at some later date for some other charge for the same crime.
This is how it should be everywhere in America.