[pseudohadamard]

  I think mainly it was, that they became civilized/baptized and christians were still free to plunder and enslave non christians, but not fellow christians.

That's what made them so successful in raiding England. The churches collected all the wealth and no-one dared question it or touch it because it belonged to God, or at least his ground staff. Then the (pagan) vikings, who had entirely different gods, saw these convenient concentrations of wealth in one place and helped themselves.