I do agree that sacrificing everything for the good future of kids and his people genuinely the best thing a king can do but holy shit, does harald not see what he is doing? He has great faith but bad execution, boa hancock is great example of protecting its people by "Working" with the devil
Harald saw exactly what he’s doing. He’s taking all the blame of the giants transgressions in the past thousand years onto himself.
Realize what he just saw:
1. He was unable to help his closest friend because of his dream and people
2. He saw his friend was murdered and an island destroyed because a member of a clan committed a crime generations ago.
It’s quite clear why Harald had to go all in. He doesn’t want Elbaf to be the next god valley and he wants his friends loss to mean something.
Sure he may be signing up to work for a devil, but it’s become clear that his people will be safer when they’re no longer this devils enemy. And if he needs to be the sacrificial lamb for his people than he’ll do it. Remember the warlord system didn’t exist yet. At this point only being a WG member will keep them safe.