However sad it may sound it is pure statistics. In a population of 400 million you are bound to find a few psychopaths. And not all psychopaths kill. But I believe it came from within his twisted mind, had he not used a gun he would have used a baseball bat.
Psychopath, noun a person with a psychopathic personality, which manifests as amoral and antisocial behavior, lack of ability to love or establish meaningful personal relationships, extreme egocentricity, failure to learn from experience, etc.
http://en.wikipedia.org/wiki/Psychopathy
Unless his mother who seemed to know him well, had alerted officials, it could not have been stopped. But rarely does a mother go against her own child.
My thoughts and sympathies are with the family members. May they find a way to heal.