This is a picture of an injured student from the Virginia Tech shootings.
I always hear the argument, "Had more students been armed, it wouldn't have been so bad", but they fail to include in their theory that the number of bullets flying would have exponentially risen had that been the case, and even the most experienced law enforcement or military officer can tell you that more guns means a higher probability of a stray bullet hitting someone unintended even if you're well or expertly trained. So not only would you have the shooter, in this case Seung-Hui Cho, with the armament that was in reality able to kill 32 people and wound many more, shooting, but you'd also have people firing on him. The Norris Hall location (the first of three areas where Seung-Hui Cho opened fire) provides a lot of cover, which could very reasonably mean a long and drawn out shootout with stray bullets heading in multiple directions. Not only that, but most of the buildings in that area were occupied on that particular morning. Bear in mind that in 9 minutes over 170 rounds were fired from Seung-Hui Cho alone, and the ammunitions were hollow point (making the impact much worse).
The Virginia Tech case clearly illustrates why guns should not be allowed on campus.