Not much to debate here really. Even if the guy had physically attacked this old man, self defense isn't usable since it was a gun free zone and he had a gun, which he then used.
Old man is in deep trouble, and should have known better considering his background.
It is a horrible event. I feel bad for the family of the victim. I feel bad for the family of this old man. It never should have happened, and if the older man did indeed report the disturbance of the texting, the theater staff should have asked him to leave or to move in order to diffuse any potential situations.
It is what it is. Not much more to say...