Well, he's the protagonist of his story though. That's why it's still a leading role.
I had the same question as you, like, “Wasn’t the Priest also a villain?” But then I realized the type of villain they’re talking about is someone whose entire role in the story is to cause trouble and hurt others—a true antagonist who exists solely to thwart the protagonist.
That’s why I think the Priest role is different from an actual villain role. The role the MC is going for now is a true evil character—rude, cruel, manipulative, the whole shebang. But the Priest's actions weren’t driven by evil intentions but by a misguided sense of justice. He did terrible things to evil people because he genuinely believed it was for the greater good. That technically makes him a villain but not a "villain character" in the traditional sense. His purpose wasn’t to hurt innocent or good people, whereas the role the MC is choosing now is a straight-up a-hole—like that black-haired guy with the funny face who stepped on MC's foot—someone who’s evil simply because they can and WANT to be.
I guess technically the serial killer priest was an antihero, but I would still kind of consider him a villain role lol