one big one is that Cheradenine doesn't remember what exactly happened to Darckense. He remembers that he turned her body into the Chair, but he doesn't remember what happened right before that, whatever it was that killed her
this is because the book doesn't explain what happened, exactly, so giving him kind of amnesia sidesteps the issue of if he killed her or if she died naturally
another is that Cheradenine is far more of a traumatized, guilt-ridden, complicated person than the unfeeling, ruthless person a lot of the characters see/present him as
that's more of a partial headcanon, but I put some of the assumptions Diziet Sma makes about him down to her not knowing really anything about his past and her assuming his comfort with violence means he's a psychopath
he isn't, he's got plenty of feelings going on in the book and he feels very bad about the shit he does during work, but she sees him as backwards and assumes a lot based off that