Now that this reminds me, I wonder, was it explained in the novels on whether the royal princesses were evil since the beginning or became evil due to the influence of their father?
I remember Keyaru making comments about the princesses that even after the brainwashing they still would let anyone die for their own or Keyaruga's profit without any question or hesitation. As proof that they are selfish and unempathetic at the core. And it would just take a little push and the right situation to revert them to the cruel princesses they were before.
From the wiki:
"Keyaru has noted that despite her memory being changed her personality is still the same in regards to how she views all other humans besides Keyaru and those she has come to care about, therefore she doesn't mind or hold it against him when he kills people who deserve it."
"Despite her amnesia, Freia still harbors some traits of her former personality, such as her talent in manipulation and lying."