By Zahira Torres and Yesenai Robles, The Denver Post, Dec. 14, 2013

[....] was a gifted debater and might have been bullied for his beliefs, according to students who knew him. [.....]  Arapahoe County Sheriff Grayson Robinson identified the gunman as Karl Pierson, an 18-year-old student.  "He had very strong beliefs about gun laws and stuff," said junior Abbey Skoda, who was in a class with Pierson during her freshman year. "I also heard he was bullied a lot." [....]

Pierson also appears to mock Republicans on another Facebook post, writing "you republicans are so cute" and posting an image that reads: "The Republican Party: Health Care: Let 'em Die, Climate Change: Let 'em Die, Gun Violence: Let 'em Die, Women's Rights: Let 'em Die, More War: Let 'em Die. Is this really the side you want to be on?" [....]

Also see:

Motive behind Arapahoe High School attack remains murky
By Jeremy P. Meyer, The Denver Post, Dec. 14, 2013