Guns being "legal" or not won't matter. Criminals will always find a way to obtain them. Gun control only keeps the guns out of the good guys hands and empowers the bad guys.
Imagine if one off duty cop or civilian with a ccw was in the Colorado movie theatre? The story would have had a very different ending...
Also, Americans are typically more violent than most other countries. That's due to a variety of reasons. Guns are not one of them.