I'd just like to point out that the vast majority of depressed or otherwise mentally ill people don't engage in mass murder. We are far more likely to turn our feelings inward on ourselves. Also, most of us have the sense to get help or accept it when offered. Apparently students and faculty reached out to the Virginia Tech killer and he refused to respond to their overtures. He made a conscious choice to remain isolated and shut down even after being asked to join others on regular social outings and having a teacher take a personal interest in his well being. I don't know what happened in his life to shut him down--but he chose to remain that way.
I wouldn't like to see either depressed people or South Koreans stereotyped as a result of this tragic event.