The only "mass murder" case I was involved with was in Red Lake, where I knew the grandfather, a cop, but not the boy.
His grandson had been abused by his alcoholic parents and after they died, was taken in by grandfather and his common law wife. That boy was depressed and on medicine/counseling, yet although anti depressant medicine helps relieve the depression it also can exacerbate impulsive behavior, including paranoia and violence. In other words, as his depression lifted, the self hatred he felt, and the hate/love for his parents was turned outward and projected on others...the boy was given school leave for his mental problems, but this left him free to surf the violent nazi type internet sites which encouraged the idea that violence against the enemy was okay.
At the local high school, ridicule was a major problem (I once gave a girl a six month excuse to get home classes because she had a skin disorder: so she wouldn't be ridiculed). So he first killed his grandfather, got his bullet proof vest and gun and then went to shoot the highschool classmates who he hated.
I won't comment on gun control, which I favor, because it is merely a cynical political football....New England's gun laws are strict, and you have to be over 21 and pass a test to buy a handgun, and then you have to learn how to use it. Laws only work when they are enforced...
The only answer to such things from those of us far away is prayer, so theAnchoress had a podcast of prayer for the dead.