I don't feel any sympathy for the priest. Had he stated his true level and thoughts, the hero might have decided to let him stay in the party.
But instead, he decided to keep everyone in the dark about his motivations and true dangers, tried to manipulate the party into following his plans and now is suffering the consequences.
The hero isn't an idiot, he's naive. But instead of fixing his naivety, the priest decides to take a shady path. The hero realizes this and kicks him out of the party.
If I were the hero, I would probably have dealt with the situation in a similar fashion. What's the point of keeping in your party someone you find suspicious?
As the adult here, the priest was the one supposed to gain the kid's trust and guide them the right path. He kept every important information to himself and did things behind their backs.