I think he's jealous.

“Meliorn?” said Clary. “I’ve met you, haven’t I? You used to go out with Isabell…”

“... I intend to marry Agatha myself. She may be a thousand years old, but she m…”

“He had become a monster. You just couldn't see it...because it wore the face of…”

“Our own Sleeping Beauty. Who finally kissed you awake?”