Dark Knight Rises killings in Aurora

[Kriven]

Let's say Holmes had been (legally) limited to a handgun and a bolt-action rifle.

He would have had the choice between resorting to the black market for a weapon more capable of killing large numbers of people (and as a result, may have acquired something more powerful than a semi-auto AR-15), and using explosives to kill as many people as possible.
Perhaps, he would have planted timed explosives in the theatre, then walked out and waited for them to detonate so he could start sniping people as they ran out of the theatre?

Also, I don't see what is wrong with normal people owning powerful weapons like assault rifles and auto shotguns, to defend themselves and their families. If I could, I would defend my home with the most destructive weapon I was able to wield.

The weapons blackmarket of the American continents actually exists almost exclusively because our gun shops can legally sell ridiculously powerful firearms that the other countries have realized is batshit insane to give to civilians. The North and South American blackmarket on firearms is literally picking up these weapons in the US and shipping them down through Central and South America. Finding a blackmarket arms dealer with weaponry more powerful than what can be purchased by civilians in a gun shop is a difficult chore.