Not surprisingly, photos from the island show numerous rescuers with submachine guns on their shoulders. The mass killing ended when good people showed up with firepower of their own.
Exactly the same happened in Virginia Tech. As soon as police fired on the culprit, he ran to the nearest bathroom and shot himself. I suspect a single person firing back from a classroom would have had similar effect even if not hits were obtained.
Prey animals do not become safer from predators if they are denied horns, quills and claws. Lawful humans are not safer if disarmed by law or custom.
PS: Norway has responded reasonably to threats before. Hopefully, they will treat human threats similarly to how they treat ursine threats.