Virginia Tech Shooting, who's responsible?

I think it's natural for us to want to assign blame to someone, and since the person who is really responsible is gone and cannot be held accountable, it's easy to blame others. It does look as if quite a few mistakes were made, and it grieves me, but I also feel compassion. Those who did make mistakes will likely never forgive themselves, but honestly, how does one deal with such a shocking and horrific event? I think many people would freeze up and not know what to do. It would be nice if some protocols or practice drills or something could be put in place to help people respond better if violence occurs.