They did mention in the manga that he saved that country but the people did something(i kind of forgot) so then he turned evil
Spoiler ish!!!
My memory is bad but I think the people started fearing the person previously blessed by the dark spirit king because they found out about the it's true powers and how it could destroy everything and stuff. So even though the person saved the world, people ostracized them because they were scared of his powers. So in turn, the person turned evil because of all the negativity he received from everyone.
But in my honest opinion, I think that the previous person blessed with the light spirit king had something to do with this. Idk, the story smells pretty fishy.
the person who previously had the blessing of the dark king became a horrible because of the people around them that's why they destroyed everything. If anything the parents are just trying to repeat history.