Hm, I guess I can wager a hypothesis on why Jem was in Envy. He envied the relationship you had with your mom, envied being the heir, envied being loved. Cas compensated another way, a somewhat healthier way. Yet Jem, he didn’t have a support system, didn’t talk to you as often, so, he just got to stew in that hate. Maybe he’s not entirely irredeemable if the proper actions are taken. But, at this point, I doubt he’d want to change, nor would that even make things better. Just make saying goodbye harder when you sacrifice him