Revealing mistake: When the Abbe du Coulmier comes running around the corner towards Marquis' cell during the fire scene, you can see him smash his head against the wall.
Revealing mistake: The first time we see Madeleine's corpse in the chapel, under the white sheet, watch her chest closely: you can see her breathing.
Answer: Because he apparently had to work the courage up to say whatever it is he wanted to say. He was conflicted in his feelings for Maddie. He's a priest and isn't supposed to 'want' her the way he does. Catholic priests take a vow of chastity and he was trying to deal with his feelings as a man and his vows as a priest.
Shannon Jackson