No need for agonised puzzlement. It's simple. Lanza was a psychopath. Many are. Difference is, he lived in a culture that glorifies guns.
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@dibblego How nice of you to imply that I'm not only incredulous but trying to persuade you. I know better than to waste time on either.
@TheColonial "I'd be surprised..." is a fallacy known as "argument from incredulity" Persuasion is implied by the nature of the discussion.