The mother was a total crackpot, her Twitter account is nothing but lunatic raving about committing suicide, mass death, etc. She likely drove the entire family insane, drove the kid to kill everyone just to make it all stop.

Her Twitter is still up, it is just her first and last name but it is being hammered and will likely be removed soon.

That kid should have never been able to access that firearm. Say whatever you want about the right to bear arms, he was severely special needs and there should have been precautions taken to ensure he did not have access to a casually lethal weapon.