So the guy was booted off the debate team for some reason or another, and he was going after the librarian that also ran the debate team. He was rabidly anti-Republican and was known to wear USSR sickle and hammer shirts to school, as he considered himself to be a communist.
http://www.denverpost.com/news/ci_24721367/arapahoe-high-gunman-held-strong-political-beliefs-classmates
http://online.wsj.com/news/articles/SB10001424052702303932504579258820517724030
No word on any mental illness but it's claimed that he was bullied. I'd like to know why this is happening so much in the Denver metropolitan area, that's 3 major ones now that I can think of.
As for the trend of mass shooters, here's an interesting take on it. I know the source falls in the whack-job category, but once in a while they are able to put together stuff that can actually be (mostly) verified, like here:
http://www.infowars.com/media-works-to-keep-mass-shooters-profiles-secret/