Let's take a look:
• Severed head? Check (6:28).
• Beatings? Check and check (14:65; 15:17-19).
• Lengthy execution scene? Check (15:22-39).
• Threats of fiery punishment and death by drowning? Big check (9:42-48).
Oh, and don't forget the demons, exorcisms, and possibly sexy dance (6:22).
So why just PG-13? Well, Mark doesn't get the full-fledged R-rating because the story is short on details. Parents can rest assured that they will find very few descriptions of blood, gore, sex, or nudity that are at all vivid. Mark is no Rita Hayworth. And he's definitely no nearing NC-17 Mel Gibson.