Irrelevant. The cause was not the point but the viability of success in a country with liberal gun laws.
Whether your spree is driven by politically ideology and was planned or a heat of the moment random attack, the availability of powerful firearms is going to make your mission a lot more deadly.
The cause is entirely the point (In this case)
The Breivic case should not be compared with other spree killers for the purposes of this debate.
"Illegal" Firearms are readily available pretty well everywhere. Breivic used "Legal" firearms in his assault because they were available to him, but with his degree of determination and commitment he would have had no real problem getting hold of "Illegal" ones had "legal" ones not been available.
This does not compare with the typical spree killer and certainly does not compare with this particular school shooting.