I'm sure half of you called it, the hero is actually the old ruler of the kingdom, here to take it back from the curse. Having survived the last century of exile by using that life-steal skill on monsters to extend his lifespan.
You also find out that he was an ethical necromancer too. The skeletons in his army are legit just mindless corpses that no one really minded going missing, and the undead Dark Knights are actually given free will and only raised with their consent (until he was kicked out and their freedom was lost).
That and the curse that ravaged the land after he left was literally a protective mechanism for the kingdom. It's supposed to turn the land hostile against any enemy of whoever wears the Crown on their head. The king/queen was literally using it to make the land hostile against not only invading armies, but also roaming monsters, disease, etc. Getting booted from the kingdom basically allowed the curse to go wild and become hostile to EVERYTHING.
Sooo, technically he's a necromancer if not referred to as one explicitly, and he's very much a good guy. He was just misunderstood beause using necromancy and life-stealing spells to become effectively immortal scares people and attacts pucky hero type people to defeat you.