Cornyx says "only one terribly afflicted, or burdened by deep sorrow, could begin to understand this" when offering him grave warden tome. It seems to imply one has to go through heavy suffering to comprehend the nature of dark. While Karla might have gone through bad stuff in the prison, there was certainly more grim stuff in her past, as hinted by the description of Dark Edge, potentially implying matricide at Karla's hand. Karla also said she used to be consumed by the lust for power. There's a lot of mystery about her.