Why did he shoot?

The "Colorado shooter" went to a theater and shot at the crowd, resulting in 12 deaths and 48 injuries. Thank God it wasn't more. It seems his weapon jammed. Thank God!

This person had been stockpiling artillery in his apartment for months. He booby-trapped his apartment before leaving to go to the theater. Where was he planning to go afterwards? The police were smart enough not to enter through the front door. How did they know? If it was known to the authorities that he had been buying weapons, ammunition and explosives on the Internet and having it delivered to his address, why wasn't anyone following him around? Why couldn't this whole tragedy been averted? The authorities knew about the weapons, why wasn't anything done BEFORE the tragedy?

We live in a very disturbed society. Violence is everywhere. It seems that courtesy and compassion have disappeared.

Everyone is wondering why he did it. I'm not sure this is the right question. My question is, why aren't more people going ballistic?