Logic:: Opposite scenarios

If someone proposes a scenario that proves their point, look at the opposite scenario and see if it still holds.

"You never see violence at a gun show because everyone there is armed! No one would dare to cause gun violence!"

Therefore,

"The most dangerous place to be is at a book fair, because no one is armed and therefore people with guns can have their way!"

In the previous example, is gun violence therefore increased by a lack of guns, or could there be another reason?

(Note to any readers- if there are flaws in this logic please comment and point them out. I'd hate to "luff" a false premise)