I do know what you mean, however I propose a slightly different scenario based on the fact that the companion was shot as well. I bet the deceased or his companion was on his/her cell phone at which point the shooter took umbrige, and was most likely impolite about it. Upon being called out the cell phone user and companion became agressive towards the couple that wanted them to behave in the theater. Facing two agressive people caused the shooter to fear for his saftey (or at least have a plausible arguement), thus the shooting of both. This also may explain the shooter being detained by other unarmed patrons.
If you’re carrying, you have an extra obligation to avoid conflict.
That is, if you were raised in a civilized culture.