Well, at least this wasn't one of those cases where all the neighbors of the murderer say he was quiet but otherwise unremarkable and nobody suspected a thing. This kid was obviously troubled and lots of people knew it.
I think it's mostly hindsight to try to find someone to blame for the fact that he wasn't restrained in some way before -- although I have to wonder how he became a senior if he never said anything in class. You wouldn't want to live in a society in which every troubled kid who acted strange was watched like a hawk and maybe put in preventive detention lest he be a mass murderer. There are a couple of things few have remarked on that seem worth mentioning.
Seung-Hui Cho was an (obviously) extreme example of a couple of strands in American culture that strike me as unhealthy in general, and decidedly so in far less extreme versions. He seemed to harbor an unhealthy resentment against the rich and "privileged" (he apparently named some rich kids he thought had especially dissed him, though it wouldn't be surprising if they were unaware of having done so), and he seemed to have been utterly incapable of taking personal responsibility for his own actions and attitudes. "You caused me to do this," the "disturbing" note found in his dorm room reportedly said. Somebody else was responsible for what he did.
The mainstream media and many of the attitudes imparted in public schools and many colleges reinforce much milder but still toxic versions of both these attitudes. You can hardly pick up a newspaper without reading a story about some new report showing that the rich are getting richer and it's oh, so lamentable. Many Americans have something of a split personality about the rich, consuming gossip about celebrities and heiresses avidly without apparent resentment, aspiring more to be rich than to bring down the rich, yet eternally susceptible to entertaining envy. Envy, about which Helmut Schoeck wrote an excellent book, is easily the most personally corrosive of the sins or vices, since it can never be satisfied. It desires only to bring down the object of envy, not to bring oneself up.
There are so many examples of voices in popular and political culture urging people to blame their problems on "society" or "the man" or "the system" or "the privileged" or the whites, blacks, browns or yellows, on immigration or globalization, the corporations, the greedy rich, and on and on that it is tedious just to mention a few. There are also voices suggesting that if you stop blaming others and work on yourself by taking responsibility for your choices, you are more likely than not to improve your lot in life. But the blame-others voices seem to have the upper hand.
I suppose it's inevitable that the videos the kid made will be splashed all over every kind of medium for days or weeks. It's supposed to be a free country so I wouldn't stop it even if I thought I could, but I can't help but regret it. You just know that some troubled or disturbed kids out there will latch onto him as a cult figure and have sick dreams about emulating or surpassing him. And having these words and images assaulting you can't be helpful for those who want to maintain a healthy and open attitude toward life and their fellow humans.