The moment Duke Balzac listened to the king's suggestion, his expression changed as his eyes darkened. However, he quickly trained his emotion back to normal. Even though he didn't like how this was going, he still had to keep his appearance up in order to conceal the incident involving his niece.
If the king would to detect something strange from his expression, things would get messy. After all, King Lucius wasn't that gullible or he wouldn't be raised to his current position. Still, Duke Balzac did not agree with the king's suggestion.
He couldn't risk the safety of the students at the academy as well as his son, Tristan, who worked there. If he let Ophelia out and she turned out to be related to the dark power behind the church, wouldn't that spell disaster?
Whatever happened would be on his hands then. The duke wouldn't and would never carry such guilt because that would damage the future generation of the Chestarrow.