Kevin Spacey's character in Seven summed it up quite well when he said, 'It's more comfortable for you to label me insane.'
People, in general, have a hard time accepting someone with rational and clear thinking could do such a thing. Mostly, it's due to the fact we view things from our own perspective and cannot imagine ourselves doing the same, unless we were mentally ill.
Not really.
Somehow, doing this 'made sense' to the shooter.
Too much planning and forethought and loiter time beforehand for an impulse act.
Absence of a viable exit and escape strategy indicates either poor planning, failure to account for some factor (like smoke alarms pinpointing his location), poor execution, or the absence of desire to escape.
Maybe he was overwhelmed with a 'sense of power' and broke with a plan which could have allowed him to leave by continuing to shoot past the point where he could escape.
(Note: many shooters suicide or suicide by cop).
Now, the mental framework within this 'made sense' to the shooter isn't what we'd consider normal, and ranges in possibility from a personal grudge against some one/entity taken out on a crowded venue to some religious motivation to seeing things and listening to the voices in his head, or perhaps an act of desperation from someone who felt they had nothing to lose.
There is a possibility of a blackmail attempt that had its bluff called, only it wasn't a bluff, but lost in what might be ordinary levels of threat traffic--the 'give me a million by midnight or something bad will happen' bit.
Discerning motive, and this seems like the act of someone who has nothing to lose, something desperate, although the person appears to be one of ample means. That appearance can be deceiving, and loss of those means lead to desperation. Whatever the case, we have insufficient data.
Without knowing more about the shooter's politics, religious standpoint, personal economics, associations, medical pathology, and other factors, the motive is impossible to discern, and whether that motive was predicated upon a specific set of delusional beliefs or cult behaviour cannot be assessed..
