I’ve been thinking about this for a while, The girls have never met Bethany (that we know of) so if she has never met them why would she hate them?
Now in the books we know that Mona was at Toby Cavanaugh the night Ali threw the stink bomb in the garage, Mona got burned that night from the fire.
We know the show likes to re imagine the story’s from the books and change them slightly, What if for the show they switched out Mona for Bethany? Meaning Bethany got burned that night the girls and Ali set fire to Toby s garage, It would explain the paintings/drawings of a blonde girl with a deformed face.
We know A likes to wear masks. Maybe Bethany wears them to hide her disfigurements.