I don't have a problem with there not being a happy ending, or with the game "railroading" you into seeing Avernus as the bad ending. One of the things BG3 does extremely well is challenge the way most people think about video game companions. We're conditioned to treat our companions in ways that would, in real life, often just be enabling shitty behavior and not being a good friend. But in BG3, doing that gets you some pretty shitty people for companions by the end of things: a psycho religious zealot, an arrogant demigod, a douchey turbo-vampire, etc. At the same time, the actual relationship between the player character and the companion characters is refreshingly dynamic, with you being able to influence people and how they think without first needing to meet a certain threshold of "so-and-so liked that" points.
So with all of that said, if the only choices are Avernus or death, I agree with the game as it originally framed it: Avernus is the bad outcome. That's the outcome where you didn't respect your friend's choices, you kept pushing and pushing and pushing--and if you romanced her, you probably guilt-tripped the shit out of her. You don't know how horrific her life was in Avernus, but you can get a sense from her post-Gortash speech. And now you've pressured her right back into that same horror, just to assuage your own feelings. You pressured her into accepting a life of pain and suffering that she did not want. If you did that to someone in real life who had a terminal condition, you'd be a real asshole.
The problem only arises when they give you such an obvious third option, and even hint at one or two other possible ones earlier in the game. And then they have the gall to put the words "we did all we could" in your mouth.
I wouldn't call people who can't bear to see their loved ones die right in front of them bad people. I don't think I would've had the heart to accept Karlach's wishes to just blow up on the pier as I watched on. Especially after I heard her lament the life she'd never have.
Sometimes being a good friend means standing up to your friends for their own good. Yes, Avernus is a place of trauma for her, but trauma can be overcome. Things are different now: She's not alone anymore. There are people who love her and believe in her. This time, the pain and the struggle are worth pushing through for the promise of a better tomorrow.
On a different note, I just refuse to believe that, in DnD of all settings, the DM can't think of a single solution other than "suffering or death" for an original character. That's just wild to me. Especially considering all the ways other characters get around their problems. Heck, Wyll can get out of his contract and save his father on a technicality. And we're not even asking for that much.
But even if we assume there was no plausible way to help her, they should've shown that to us throughout the story. Dammon's not enough. We should've heard that from the Steel Watchers and the Gondians themselves. Maybe throw in the deep gnomes, too. Then, and only then, can we come to terms with the finality of Karlach's plight.
