Her storylines endings are basically the combination of the two worst features in the narrative: 1)Larian pushing the degrading cermorphosis horrors into romance territory and 2) The overtly dark tone of the story, that doesn't allow much in the field of happy endings. Also, not thrilled that the character that seems to me one of the queerest companion of the lot, even with everyone being playersexual, has the most distinctly unhappy romance.
If you play as a good character, I don't think they can fix this just by giving her the option to survive with her heart condition, since apparently they decided that someone has to undergo ceremorphosis and well, if it's you, it's still a doomed romance, with one of you turning into a sexless soulless husk of yourself. If you persuade Orpheus to undergo the ceremorphosis, you're not only dealing a massive blow to the gith slave revolt, but also dooming yet another greatly wronged person into a fate worse than death, which is just wildly shitty thing to do.
Given the situation, I doubt we'll get a viable good ending for Karlach(not really for any romance, since getting to the end is all dependent on someone else getting degraded into a mindflayer in the end), which is a bleeding shame.
The promise of being led to death is reason enough to follow.
