Poor, sickly, and having only two teeth, the mestizo is as determined an opportunist as they come. He's smart too, and observant. Too smart and observant, if you ask us. With the insight that would make Sherlock Holmes proud, the mestizo sees the priest beneath the disguise immediately. He's not subtle, though. He calls the priest "father," not even hiding his secret knowledge. As you might imagine, the priest isn't too pleased about his presence.
He could hardly be more obvious that he intends to betray the priest for the reward. Like, dude—have you ever heard of subtlety? He obsessively protests his own innocence and purity of heart. Really obsessively. He goes on and on about it, and even scolds the priest for his lack of trust after he's betrayed him.
Like Gollum in The Lord of the Rings, the mestizo is a villain, but a pitiful one. He commits arguably the worst sin possible, but as the priest notes to himself, he's not bad as all that.