According to the prophecy in the beginning of the game, both killing Cass or agreeing with her coup are bad ideas that will lead us to a bad ending, as neither her nor the king can die or the remaining "snakes" will kill each other and you will fall along with them... I just don't know if the third "snake" is the mother or Ellin, the cradle remark makes sense for both right?
On the other hand, Cass just letting you kill her is really suspicious, she probably has a plan to restore herself somehow as she wanted her body taken by her servant so perhaps this decision isn't as final as it seems and she is just pushing you to see how far you will go and adjust her plans accordingly.
On the other hand, Cass just letting you kill her is really suspicious, she probably has a plan to restore herself somehow as she wanted her body taken by her servant so perhaps this decision isn't as final as it seems and she is just pushing you to see how far you will go and adjust her plans accordingly.