So Columbine was who's fault, now? It wasn't the shooters' faults?
It is the inaction of responsible people that are ultimately at fault.
When you look at all the cases of school shootings, there are warning signs EVERYWHERE. No one wakes up one day and suddenly decides to shoot-up a school.
Same thing with the kid at Virginia Tech. He was posting hate videos on the internet and creating very violent literature for months before his rampage. Many people knew about it, and did nothing.