He started failing at the one thing he was actually good at and the place he 'fits'. (it does make one wonder if Ca Universities are up to the same standards of others). He didn't want to die, he just want to be successful at something and he didn't want to go back to living with mom and dad. He wanted to hurt some people for all his long felt hurt and his failure. He is now the mass killer who has killed and maimed the mostest and he won't be living with mom and dad.
Furthermore, he then admitted to police that his apartment was booby-trapped with explosives. If you were really an evil-minded Joker trying to kill people (including cops), why would you warn them about the booby trap in advance? It doesn't add up.
It is also known that he left Techno music blaring in his apartment on loop. There are no reports that this was business as usual. I suspect he started out thinking somebody would complain or the mng or whatever would set it off and add to his mass kill numbers. I think he may have been sickened a bit after he shot up the theater. He stayed at his car near the scene and confessed what was at his apartment and offered no resistance.He also had the ammo to do MUCH more damage than he did. Perhaps he was beaten by himself by suddenly becoming overwhelmed with conscience, not the fictional charter of the Joker. Though the past Batman movie did make the Joker have reasonable points.
"Holmes was taken into custody shortly after the shooting, police said, adding he didn't resist when he was arrested," reports a local CBS news affiliate.
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I also wonder if he didn't start off planning to shoot up his school but his access was removed once he started the withdrawal process. He didn't bank on that. He sat on his roof top and went to a local bar, always with his backpack in tow. I suspect he was plotting and gathering his nerve all along...
Batman is when he finally held his breath and stepped off the diving board. It was dark, that made it easier to go against his conscience.
The music had been going for some time (hour?) according to one of the other threads.
My guess is the plan was for the IED in the apartment to detonate and draw LE/FD/EOD there and give him a few extra minutes.