To use a metaphor that I don’t know whether @poorquentyn will agree with, Euron is the Saruman of ASOIAF rather than the Sauron of ASOIAF: he’s a magic user who fell to the temptation of power without the constraints of morality, and stared too deep into the palantir, until he was seduced by Sauron.

In other words, he’s not the main antagonist, he’s the intermediary antagonist the heroes have to deal with before they can get to the main antagonist; he’s the one who besieges Helm’s Deep to prevent the Rohirrim from riding to the rescue of Gondor, not the one who besieges Minas Tirith.