Yea thats one of the big reasons most people consider this supposed 'good' ending still bad.
Every time you bring up the possibility of going back to Avernus, even temporarily, Karlach is adamant she'd rather die than go back. This never changes.
Not true,
you specifically have a option during the ending where you desperately ask her "Do you wanna stay here to die or go back to Avernus with me?"
And guess what? Karlach wants to go back with Tav.
Hell, she would also prefer to go back with Wyll too, if your only her friend.
She only seemly accepts death if Wyll isnt around as the Blade and you were only her friend.
And if she is the protagonist character, she evolves too, feeling confident that she can return to the material plane after solving her issues in Avernus.
