Could've, should've, would've. Fact is, you weren't there. I've come around to the pro-gun argument but this is a bad example for the pro-gun crowd. It's not something that could've been avoided. If the campus had a pro-gun policy, then the killer would have lured her somewhere else to do the deed.
Any age is too young to know what to do in this situation, but we're looking at a room with a bunch of 21 year olds and under, most of whom went straight from living with their parents to a college dorm. Expecting them to rush and jump onto the killer when they haven't seen anything like this in their life is a huge expectation, and frankly really apathetic to how bystanders must have felt.
And finally... I respect a college's decision to ban guns on campus. It's their private property so they can do what they want. After the various campuses I worked, studied, and lived at, I would never want to see guns part of the equation. Most of the off hours are spent getting hammered and chasing tail, not to mention the trivial drunk fights that break out between people. Let's throw guns into an immature situation, that'll help!
Campuses have a thing called security which can be called in an instant, but like RightinNYC said, there was no avoiding this outcome.