I can't help but think that many of those Norwegian kids would have lived if there had been a few responsible gun-carrying adults on the island. Unfortunately Norway doesn't allow the carrying of concealed weapons.
To own a gun in Norway, one must document a use for the gun. By far, the most common grounds for civilian ownership are hunting and sports shooting, in that order. Other needs can include special guard duties or self defence, but the first is rare unless the person shows identification confirming that he or she is a trained guard or member of a law-enforcement agency and the second is practically never accepted as a reason for gun ownership. ...
There is no apparent public desire to introduce a concealed carry permit at this point in time, and there is no such license available to civilians.
I bet that Anders Behring Breivik was the only person with a gun on the island, and wasn't too interested in obeying any laws.
(HT: The Truth About Guns.)