The look Law sends Cora at Mary's words is at once both accusing and resigned — half I know exactly what you're doing and half there's no way out of this for me that doesn't result in hypocrisy.

(If this was anyone else, he'd turn around and justify it with that this has nothing to do with him at all... but it's not anyone else, it's Cora-san, and this is important to him. Law had sworn, back when he'd decided on his plan, that there wouldn't be anything he wouldn't do for him. Apparently this is now included in that "anything".)

Instead of arguing, Law fixes Mary with a look.

"One rule: kill anyone needlessly and I won't teach you anymore."

He's not making rules about hurting people... just killing. He may not be the world's most orthodox doctor, but he is a doctor.

Then, he shifts his attention to Cora, and the other stuff on the table.

"Where did all those things come from?"