My guess is that Cho was influenced by the case of Gang Lu, the physics grad student who killed five faculty members in 1991. This incident was the subject of a recent indie film starring Merryl Streep and was related in a NY Times article less than one month ago.
As an English major and amateur playwright he probably followed indie movies and may have read this article. There are some chilling similarities.
Yep, and Gang Lu also had two guns on him, during the shootings.