well we can't have it both ways.Why? The dragon is a different species, a predator that kills to eat, its just intelligent enough to have some added vengeance
If its just a predator with just enough intelligence for vengeance....than we can't fully vilify the king for slaughtering the dragon's children and trying to kill it. Its similar to a Shepard taking out a wolf to save its flock.
Either we apply ethical judgments to the dragon's situation....or we don't.