Two points about the Virginia Tech massacre.
1. I've been surprised at the number of people who have blamed NBC for releasing Cho's videos. There is the classic argument for free speech - that while ideas may actually be harmful, it is better to diffuse and debate them out in the open, rather than achieve dubious results by suppressing them. If you do not believe this, on some level you must be opposed to free speech, since the case for suppressing harmful ideas is immediate. Either a whole lot of people are not being consistent, or the support for free speech in our society is not as near a consensus as I had believed.
2. Now that the material is out there, we have a much better idea of what went on. This knowledge may be helpful in figuring out how to prevent incidents of this form. This article in Slate, contrasting Cho with the Columbine killers, is particularly useful.