THAT is the world the game is set in. Why SHOULD your Tav succeed when saving Karlach isn't even Tavs main focus? Why can't one companion die because they refuse to do the one thing that will save them? Avernus was a choice, which Karlach refused in the original version, because "It would be worse than death". I would have been fine if the writers let her die with dignity after she saved Faerun and was brave enough to choose death over servitude, but now the fan-service is done and y'all can have an ending where Karlach lives and fits with the world that Larian built. What people are asking for now is the unreasonable and the unplausible.
This game is a choice based RPG, and Tav is not some prebuilt character like in The Witcher who has their own predetermined wants and goals. Tav is MY character, and I determine what his main focus is, not some arbitrary template that you have thrust upon them. Also, Larian didn't build this world, it is a world that they were given which has its own rules, which for the rest of the game Larian follows without issue. The fact in this one specific case they don't is not a good design choice. In fact it is so egregious I can only assume that it wasn't intentional and was not a design choice, but simply a matter of lack of time.
Finally, it's very disingenuous to attempt to imply that someone has mental health issues because they disagree with what you're saying.
