Rational doesn't mean "thinks like a typical person" (or more often "thinks like me"). Rational means "able to follow some consistent set of rules". All politicians are rational (just highly self-centered). Wanting a place on a scoreboard is one possible value that allows mass shootings. Another is feeling wronged by "normal" people, or people associated with some particular group, and seeking revenge. If the media didn't make up the "what's your religion? Christian?" thing, this shooter also exhibited the latter. Provided that you aren't miles away from civilization? Merely an inflated hospital bill, at least in America. But that's a different problem.What else does a gun imply outside of a gun range, hunting ground, or in the hands of police officer?
Then what is irrational? Rationality is defined by the society in which one resides. For example many people look at the middle east and see laws among numerous countries there as irrational, but that is because it is in comparison to our own set of rules. Politicians act irrational by denying that climate change is not a real issue when 99% of the scientific community agrees it is. In US society it is irrational to want to kill someone for no reason. In most place on the earth it is irrational to want to murder people. There is no rationality to mass shootings or mass shooters, so we cannot expect them to act in a rational way by any means.
Rationality is the quality or state of being agreeable to reason. The idea of committing mass homicide is not reasonable and anyone that could come to the conclusion that it is an answer to any question is being irrational. What I should have said was no "good" reason. For example if I kill someone who is trying to kill me. I've still killed someone, but with the justifiable reason that I was defending myself with an equal amount of force.