Originally Posted by mattmcrich
Karlachs whole breakdown after killing gortash is absolutely heart breaking it had me messed up after watching it. I think the reason she turns you down until the end is because she's so scared of hell and falling into zariels hands again and being alone again. But by the end of the game she sees she has a life worth living again with friends and partners who love her. That drive to live and keep living her new life is what drives her to be able to go back to avernus. At the start of the game she's very much driven by revenge and getting back at those who wronged her and when she kills gortash she feels hollow and needs to look inside herself for answers. That's my interpretation at least as of now.

Yep, you got it. When the hollowness of her revenge sinks in, it's her lowest point in the story.