The shooter was mentally unstable
he didn't say "oh hey let me go kill some kids today
because im really happy"
calling him a waste of human life or
saying that he shouldn't have even lived is cruel and just
plain rude.
he needed psychological help and he didn't recieve it.
he didn't ask to be psychotic
understand the reason better:
http://jamesjmurray.wordpress.com/2012/07/19/why-do-people-kill/
http://www.scientificamerican.com/article.cfm?id=anger-management-self-control