Except she literally doesn't want that. She wants to cure herself and be free and if that fails, she'd rather die than go back to avernus. And the problem is that despite many potential ways of actually fixing the issue we're not even allowed to TRY any of them. I'll leave out cheesy ways like scroll of true resurrection and all, but Gondians, Steel Watch or even Dammon should be able to figure out something. If not, why can't we get a deal with Raphael, Mizora, Mystra or other god or even Zariel herself? Either do something for them or promise a favor in future? Hell, maybe we could just buy Karlach's freedom with Soul Coins, but we'd have to get our hands a bit dirty to get enough of them? Can you use any argument why we can't do anything out of the listed things? And please, don't use "real liferino not fair tragedy so good" because we are already able to bail out a guy with nuke in his chest who was ordered a suicide mission as the only way to redeem himself by a LITERAL goddes of magic so that argument isn't really working there
