I get it. Just understand in my particular case for me this choice was: help Karlach with her engine and let her go to hell or the ending with her death, and I was quite satisfied with what I got. In your case (and most other people here, as I understand it) the choice is only one: Karlach's death or Karlach's death.

And as for how she could be given another clearer "or" choice, I think it could be tied in with Wyll and her devil, especially given their relationship with Karlach stretching through the game, something along the lines of Wyll sacrificing himself for Karlach or making a new pact with his deviless for her, I'm not sure, a thought that just came to mind thanks to your breakdowns.