[ by Charles Cameron — where motive is a multiple choice question, all (or several) of the above may be decent responses, but i’m checking “dunno” ]
The waters haven’t settled yet, and we don’t have a clear picture as to the shooter’s motive or motives, yet.
One recent suggestion is that Mercer left a “manifesto” (somehow, that’s what these things get called) in which he claimed “I am going to die friendless, girlfriendless, and a virgin.” That’s reminiscent of Elliot Rodger, the Isla Vista killer from May 2014.
But he appears to have studied several mass killers, and maybe (with an assist from news, social media & 4chan guesswork) left “clues” pointing in enough directions to keep us all talking about him..
friendless, girlfriendless, and a virgin
and so on..
Please note, these links were hastily gathered to make my overall point, and better ones may well be available — the amount of coverage and speculation is vast.
FWIW, Bob Shea and RA Wilson pillaged the many conspiracy theories sent to Playboy’s legal department and amalgamated them all, however mutually incoherent, to form the Illuminatus Trilogy. Wilson later wrote that his basic assumption was:
Suppose all these nuts are right, and every single conspiracy they complain about really exists.
Guns? Mental illness? No comment at this time.