It was the murdering that was illegal - the rest was just window-dressing.
Ah, I see. So even though there were laws against carrying concealed weapons or bombs on campus, these two happy campers at Columbine decided to break the law and kill a bunch of people.
Even so, many people on this thread believe it is against the law, or against common sense, to allow kids of any age to have any access at all to firearms. Even though the law in many states allows kids as young as twelve to have control of a firearm.