A pity he didn’t terminate the murderer with extreme prejudice - It would saved the state of Texas time, trouble and money.
Certainly would've been preferable. All he'd have to say is that the suspect advanced on him in spite of the weapon, but there's always a chance of one or more witnesses somewhere. In fact, I'd guess that the yard was probably full of witnesses by that time.
But another reason would be that this man could be judged incompetent to stand trial, thus keeping him around and even eventually freeing him when the powers that be judge him mentally fit for life as a free man.
I will hope for the best: That he has a date with destiny in the form of a needle in Huntsville.