I think the difficult thing here is there is no answer. Even if he and his mother were alive to tell us what happened, how could there be a "why"? At least one that makes sense. 'Cause it doesn't make sense. He was crazy. We could find a more PC word, or a clinical diagnosis that someone made or a thought that someone had........but at the end of the day, he was nuts.
I am not saying he wasn't troubled, or didn't come from a broken home, or might or might have not had Aspburgers or whatever. There was some portal in his brain that said this was ok.
I bet we all have stories or circumstances. (I sure do........I was joking the other day, that my life was too nuts to even qualify for "My Life is a Lifetime Movie".) There is a brake that normal people access. Murderers, for whatever reason, override "normal" and replace it with totally self serving rationalizations for what they want to do.
There is (IMO) bad and evil in the world. This was a broken person. And, unless there is a vaccine available to prevent it, searching for a reason is a futile action. He did it because he could. He did it because he wanted to. He did it because he was angry. He did it because he wanted to be "heard" and felt he wasn't being. And, he did it 'cause he didn't care about anything but himself. What more is there to know.
BTW - I think the same answers apply to Susan Smith, Drove her two kids into a pond and left them to die.
Seung-Hui Cho Virginia Tech
Harris & Klebold Columbine
Charles Roberts - Nichols Mine
Lastly, IMO anyone who murders anyone (not kill, but murders) is nuts. We search so hard for answers, but it is incomprehensible. And, frankly, I would worry more if I did understand.