Cressida was trying to understand Hartley as he interacted with Ashton. She could learn a lot about him while he was there, and then figure out his motives. She felt protective of those in her house and naturally so. She didn't step in unless she had to, understanding their desire for independence and their self-sufficiency. Ashton telling her about Blake had made her think him a coward, but there had to be a reason he put him at St. Margaret's. Perhaps he wanted to kill a full developed dragon and one that knew what it was doing. But Ms. Menides would know this, surely, and never allow it. Would she? The fury was often difficult to read when it came to intention.
"Not natural?" Cressida repeated and then said, "So whatever is happening to him is completely the reverse." Which didn't help as much as she thought it would. Saying something wasn't natural was closing one door—the door that Cressida knew a lot of answers to—and kicking open one that she couldn't really help with. It would explain why her efforts, along with Barclay's and Kiakos, were met with such dismal failure.
At the request of the knife, she felt a similar react to Mr. Pheres, but then said to Mr. Pheres, "We are not in a position to decide whether or not we can trust him with a knife." She turned to Hartley though and said, "Unless you are the only one capable of this, it might put a number of the staff here at ease if someone else does this. While you were called in when we ran out of options, a number of staff here do not trust you."