What makes something a monster? The way it behaves? Certainly anything that kills for no reason other than to snatch life from another living being can be described as monstrous.
Its physical appearance, perhaps? A creature that’s hideous to someone of a different species might be considered the height of perfection to others of its kind. Beauty, as humans say, is in the eye of the beholder.
Then again, something that looks beautiful can be malevolent, even deadly. Appearances are deceptive; you can’t always tell just by looking.
The Doctor sees a monster every time he looks at his reflection.