All the facts aren’t in but it appears a madman dressed up as a policeman and took a boat to Utoya Island where he ordered a group of teenagers to gather around him, and then started shooting. The death toll has reached at least 80, but they’re still counting. The only thing that stopped the killer was other people with guns, in the form of a SWAT team that also had to take a boat to the island. Apparently, the killer had his way for quite a while since he had the only gun on the island before the SWAT team arrived.
If some of the adults on the island had been armed would the death toll have been lower? I think so. The shooter in Tucson who recently killed 5 and wounded a U.S. Congresswoman was stopped by a CCW holder (without using his firearm, but he was armed).
It may be that the United States and Israel are the only civilized countries where a majority of people understand the simple concept that a deranged man with a gun can only be stopped by other people with guns (unless a brave group takes him down by charging him at great risk to themselves, which also happened in Tucson). This photo I took in the City of Acre, Israel in 2002 of a couple of school teachers on a class outing with their students reveals that understanding.