I think it's a very jarring tonal shift to have her lusting for life and deathly afraid of returning to Avernus most of the run until the last act where she does a complete 180 and becomes a doomer who's suddenly fine with their fate and fine with going back there.
Okay full disclosure I was already in the middle of Act 3 when Patch 2 dropped, so I'm not aware of whatever lines were added in Act 2 or at the start of Act 3 and can't say anything about the general 'doomer attitude' or not, but I'm not seeing any of "suddenly fine going back there" you mentioned? I am currently right on the threshold of the final battle and by this point she's still
adamant about dying rather than going back (in fact briefly being back gave her a panic attack and a determined "NEVER AGAIN I'd rather die in the world I love than live in the one I hate" which is pretty consistent with everything she was saying before). And the opportunity to convince her otherwise at the very end was there before the patch I'm pretty sure, so that also didn't change.
Also considering the post-Gortash scene and the current absence of solutions I think the 'doomer' shift is appropriate. Even people who manage to live in the present and find the little joys in life start to break when death is quite literally breathing down their neck with no hope in sight. She has to stuff all the negative feelings inside to enjoy the positive ones while she can, 'cause that's who she is - but they're still there, and they start to show. Seems like a natural progression to me - made only more heartbreaking by the contrast.
(Although I will say, I'm also not seeing Avernus as "well she loves action and adventure, so it's a good way to check some stuff off of her list". The way I see it, multiple things across the game (in banters, in her origin's dialogue options, etc) imply that in actuality Karlach very much longs for a quieter/simpler/normal-er life down the road, but it's 1. unfamiliar and scary because she hasn't had it for so long and doesn't feel like she could fit in all that, and 2. not something she can really afford to think about, 'cause it's a life that involves time and future she's lacking. (Also 3. not something she readily admits 'cause looking tough and ready to throw down is something that kept her alive for more than a decade - like an armor.) So instead she opts for collecting as many adventures and experiences as she possibly can in the short time she has - but had she had a choice, it would hardly be constant fighting. Not in the long run, anyway. But that's just my interpretation, to be fair.)
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