Not to mention that he darn well could have sat somewhere else, away from the offensive texter, after leaving the theater to complain to management.
IMHO this is part of what's wrong with society. He had a right to sit there. Did the younger man really assault him is the question.
Good shoot? Doubt it. But this whole thing is being made into more than it is.
Old man had the right to be there. Younger man probably got in his face after tossing popcorn. He overreacted and shot him, thinking he would be assaulted (or after having been assaulted).
Duty to retreat is an idea that should die a fast painful death.
And just to clarify, I mean that in this fashion.
If the guy had said "you come back and I'll kill you", well then, we know that coming back will result in an escalation of force.
But, if he didn't expect to be assaulted for returning to the seat he had held before, then how is he wrong to do so?