2 things - 1) It was reported that the texting itself occured during the preview trailers, not the movie itself. I've often done that myself, check e-mail and and such during previews, share the information, coordinate schedule with other people who may be attending the movie with us as well. I do agree that once the movie starts, then the phone needs to be put away and put on silent.
2) Other outlets are now reporting that the argument was escalated by the shooting victim, with him yelling at the shooter for 'telling on him', then escalated it further into the physical realm. I suppose at that point I can see how the older man (71) was feeling physically threatened by the younger (43) and felt he had to take immediate action to protect himself.
I'm not saying that's exactly what happened here. I am pointing out that information is still being developed and details are still coming out. It's forlorn hope I admit, but I'd like to think that we could all dial down the outrage until all the facts are learned, then castigate the guilty party(ies) and not just vent uncontrollably based on our own immediate perceptions.