Karlach is, like, the one remaining element of some of the ideas that were present in early access but discarded prior to release. I appreciated her ending and thought it was moving and bittersweet, but it's also a joke that she's the one character who gets the short end of the stick.
In early access, BG3 had a thread running through it of people being unable to accept their present situation and seeking ways to escape it that only made things worse. Auntie Ethel is the most obvious example, a whole quest about it. Gale's magical consumption is another obvious one, where it was much more difficult to feed him three items and he would often end up making a deal with Raphael (can he even still do this?) But there are other little elements, too. Asking Volo -- a man with no medical experience and perhaps a tentative grasp on reality -- to root around in your eye socket would end up with you losing an eye, and a permanent change in your stats. Making a deal with Auntie Ethel was the same. These were bad ideas borne of a desperation to escape your situation. The dream visitor, "Daisy", offered you power -- was that worth it, or was it another bad deal?
Karlach appears to be the one character who goes, no, that stuff isn't worth it. This is my situation and the time I have left, I want to make the most of it. So, she does. She gets to save the world, fall in love, and make some great friends. Then, she dies. I think this would've been really powerful in a game where a lot of your other characters had tried to make deals with the devil, literal or otherwise, and suffered for it.
Instead, Gale can fix his condition with a few cheap baubles and reclaim his place as Mystra's lover. Lae'zel comes out the other side of her crisis of faith as a full-on revolutionary. Wyll can escape his pact and save his father. Tav's illithid powers have no consequence. But Karlach can't fix her engine, can't even look into it. Karlach has to die to serve a thematic point that no longer exists. So, it doesn't feel poignant or meaningful, but just kind of mean and bizarre.
