It is your amazing verse so it is your call. It would be the logical step to kill off Castiel, but just so you know I would still read it. Sure I would be sad you killed Cass, but at the same time I know you would do it in a way that really fits with this story. Have to admit I would sort of love to read what Dean is thinking when he is done and see if you add in Castiel's voice like you did with Sam. If you do decide to do another verse, know that you have a supporter for your decision.