"I mean to care for it." Spoken as if it is obvious, yet still as amiable as possible. Kindhearted, even. "I will teach it not to harm others."

So he says, anyway. In truth, he'll only teach Glaurung to be clever and smart, for his own survival. It just happens to include not harming people. Yet. But he knows the line of Feanor (both foolish and stubborn. foolishly stubborn.) and as much as he was their enemy, it would be best not to make an enemy of them here.

"By the Elves of Aman I was named Mairon. That is the name I would give to others."