Parents certainly need to teach their children right from wrong, but I'm not sure I'd say that divorced parents can't do this as well as any others.
As more facts come out about this case it seems pretty clear to me (and a lot of other people it seems) that the gun laws in America make such an event very easy.
I'm also starting to have some sympathy for the young man who did this. I know it was a terrible thing. But he was clearly very mentally ill - it was known that he was very mentally ill long before this happened- but nothing was done to effectively deal with this. I don't think he was truly responsible for his actions. Having had a close relative who went through a short period of severe mental illness and behaved very bizarrely (but not violently) and did some highly peculiar things, now he has recovered we know that it wasn't the 'real' him who did those things. He really wasn't himself at all. It was as though his real personality had left his body and something else - the illness - was in control. I can see how some people can believe in possession after witnessing all that. If he hadn't been treated I can't imagine what state he would be in now or what he may have gone on to do, yet before his illness he was a lovely man, and he is a lovely man again. We can't blame him for the effects of his illness any more than we can blame someone with a broken leg for limping!
I do believe that some people are just plain wicked and evil in their actions, deliberately. I don't now think that was the case with this shooting. If that boy had been helped then those people would all still be alive and he would have had a chance to recover and go on to live a normal life.