The king is a good king, but couldn't protect MC position.
MC got the job of hero by his merit as trained solder and the holy power revealed in him, but no one could imagine that his aura was killing some monsters and making others run away, of course it would look like his expeditions are all failures, even more as after he leaves the area the monsters can return.
Not being a noble he had no support from the country, the king did what he could by making leave the position and give a big sum of money to retire.
the new hero is trash for faking being a good person, but he thought he had what was needed for the job and wanted to do it from what I understand, he simply didn't realize he wasn't cut for it, or possibly broke the hero taboo and lost his holy power.
Unclear if the taboo is actually real and MC bypassed by doing it with a angel and would have lost his holy power if the girl wasn't one, or maybe he lost some, but given how much he had is not enough to make him weak.