The only reason people are this salty is because the characters we are complaining about were well written and emotional investment grabbing enough to provoke such a response. If no one gave a shit about these characters, we’d be having a whole different discussion.
I very much agree that Karlach in terms of personality was highly engaging and is a big part of why I feel she got shafted hard here. That's a definite credit to the writing team.
But at the same time... this is more then just that. All the other characters, including Gale who's got pretty much the exact same condition going on (thing inside body that's slowly killing him and there's no conventional way to cure it), were given a shot at fixing their problems, and got to walk away after everything that happened. Even though in Gale's case he pretty much brought that onto himself.
So what does it say about the plot, when the one character that is genuinely a good person, whose only crime was that she trusted the wrong person and got sold into a slave army, is the *only one* that's not allowed to have a somewhat positive ending? No matter how many hours you put into desperately trying to help her, and how many various methods and favors you find that could theoretically help her.
To me that isn't a well-written tragic story. That's just the Dungeon Master desperately wanting to kill off a PC because reasons.