Everyone seems to agree there's never going to be a shortage of terrorists and wackos... and that we **usually** can't identify them before they do something crazy, no matter how we look for 'red flags'. It was not illegal for Holmes to buy all that stuff, and he had no criminal record.
What can we DO except make it harder for wackos to obtain stuff to do mass murder?
Look...babies will always chew on stuff, so we monitor kids toys and **regulate** things that might harm them! The point being, we don't NEED lead paint and small, breakable parts on kids toys.
Society also doesn't NEED many of the categories of guns and ammo they can now buy...or steal... easily! Regulation of serious weapons should not be an issue! It's just that these are BIG BOY'S toys, and there is too much money at stake!