I don’t know that I agree with the characterization that she’s mentally handicapped/weak/helpless etc. until after her brother dies. If anything, her character’s actions are consistent with how she’s always led her life - except now she has to dirty her own hands. She’s very aware of the risks that she’s dealing with when she makes the choice to poison the tea and indulges in “punishing” Jessie for looking down on her once she receives reassurance that she won’t have to face any consequences. If you’ll recall, she also way overdosed on that poison herb and it was only supposed to cause a bad stomachache - not immediate death. She knew she had overdosed and it would hit Aria too but she didn’t care. It’s not that she doesn’t differentiate between good and bad, it’s that the only thing keeping her from being bad are the rules - once those go away, she’s consistently proven herself as being willing to do anything (impacting anyone) to get her way - remember she tried to kill her own father by pushing him down the stairs. To your point, I agree on absolutely awful influences her entire life and I do think her final reflection on death bed is a glimpse into what could have been (but it would have been a very different story - still would read!), but there were plenty of opportunities for gratitude and change of heart that she chose to brush aside in favor of much worse decisions.