Asking "what if?" before attempting anything would only stop noble endeavors. Consequences don't seem to weigh on criminals.
It is the morning after. We know nothing about this tragedy yet. Let's hold off the lynch-mob blaming for now.
There is a difference between the enclosed environment of a privately owned store and the open environment of a 4+ square mile publicly owned multi-building institution. When you enter a store, you are entering private property and the owner of that store has the right to secure his property any way he wishes, and you surrender part of your rights to enter his property. When entering the campus of a public college, you still have the same rights you have anywhere else considered in the public domain including a reasonable right to privacy.
What you are advocating with your call for greater security via cameras is a violation of that right to privacy. Do you really believe that the young men and women attending Virginia Tech would ever submit to having their privacy abridged by campus security cameras. Right now, there might be a call for such security measures, but (if enacted) within months there would be all manner of protestations by the student body until those invasive security measures were removed.
Ben Franklin addressed the issue of security vs. liberty best when he said, "Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."