As humans, our first reaction to a problem is to place blame. But not everything has to have someone to blame.
This was no one's fault except Seung-Cho's. And he even he probably wasn't in complete control when these events were taking place. I mean, he was obviously VERY mentally ill. That is apparent in not only his recent actions, but from his personal computer, his plays, and his essays. He was a psycho, and the only thing that could have stopped him from doing this was if he had been sent to a facility at an earlier date, but the problem is that he was friends with NO ONE, and NO ONE had that chance to observe his behaviors.
The teachers and students that read his writings before he committed the murders couldn't just had said "he wrote this, it's weird, institutionalize him." could they? No. If they had, they'd just be accused of trying to cause problems and forgotten.
PLEASE stop placing blame, people. What's happened has happened, and the only thing we should be doing is keeping these victims in our thoughts and in our hearts.