I've been wondering exactly what that "more" is? The given story is words were exchanged, but I have a feeling the texter got hot-headed about anyone telling him to shut his phone off and immediately ramped up the tension. Doesn't justify the shooting, unless more information comes out.
The history is the retired cop has a history of harassing people for texting in the theater. Must be rough to try and deal with not being able to write speeding tickets anymore.