Double whammy of abuse and neglect which causes IMMENSE amounts of trauma.... Not to mention that Lafilia's dad is mostly at fault bc he doesn't make ANY time for her, despite knowing how abusive her mother is. Grandmother isn't really being helpful either.
So obviously that cousin brother is the most suspicious here... Maybe he thinks Meridian could usurp (?) the authority in the magic tower even though she's technically an illegal witch?
The solution bis mass murder. Literally just don't hold back. Go on a fucking killing spree.