To be fair though, Karlach isn't the only companion with a tragic outcome. She potentially has the most negative 'good' outcome, but other than Gale and maybe Lae'zel, none have an entirely positive outcome.
You are generally correct, I agree that most companions don't exactly get fairytale endings (especially when not played as origins). However, I think in Karlach's situation a bittersweet ending more equal to what Gale or Astarion (etc) get would be something like. She gets to live more, but her lifespan is still shortened and the engine will just blow up some years later. Or she gets to live but she's left permanently disabled in some way that makes her weaker. Or yes, Zariel will forever be on her tail. Like, "an end definitely better than what we met her as, but there's still a catch, even if it's worth it" is what I'd describe the general state of 'good' endings for most of them but her. She's technically back to the square one. Or even worse, since there's no time left. Arguably, the only thing that changed is her having friends/a lover willing to help. Which is where the Avernus ending comes in.
Originally Posted by NomTheBurritos
Karlach Origin endings spoiler
Oh hey, thank you! Okay, I think that actually gives some context to the scene when she's a follower as well.
When you're at a point where you have to ask yourself "Can I do this?" and instead of mulling it over on your own and having to convince yourself to go through with it you have to look into your lover's or good friend's pleading eyes as they ask you to decide "No" - there's logic in actually failing to convince yourself that you can. And if per her Origin run she can canonically convince herself otherwise, that she's better and stronger and she got this, even as she's alone, then that argument must hold even more weight when she's actually not alone. Still an abrupt turn from a whole game of denying the Avernus option, but at least it's a consistent turn. Desire to live is a thing, after all.