So every time there's a murder anywhere, even if it's a guy killing his girlfriend, we should lock down everything within a 5 mile radius just in case said murderer decides to wipe out a few dozen more people by locking them in a building and opening fire??
25,000 students at VT, plus faculty, plus staff, and there's no way you can protect them all if this guy wants to go on a shooting spree. If they cancel classes, students are still packed into dorms.
So, in summary:
1. There was no way of knowing one murder would lead to a shooting spree.
2. There's no way to protect everyone on a college campus if someone wants to go on a shooting spree.
3. WE BARELY KNOW ANYTHING ABOUT WHAT HAPPENED AT THIS POINT!