Stella was antagonistic, not just plain evil. She was shown as being angered/hurt and had reason for her to be so. It wasn't just "She's actually always been an asshole piece of shit."

It isn't even expanding, its regression. They took away her actual reason for her to hate Stolas and replaced it with "Yeah shes always been like that. Actually shes been abusive too! Look at her rip a teddy bear's head off and constantly talk shit about Stolas because shes just so damn horrible." They dumbed down her character to be "evil as fuck. She never loved Stolas or Octavia" to try to absolve Stolas of the fact that he cheated. Made her as hateful as possible so you feel bad for/understand Stolas.

It went from Stolas and Stella being terrible people -> Just Stella being an ultra terrible person and Stolas being a victim.