It was an eye-for-an-eye with a twist: Ellaria will now be made to spend the rest of her days rotting in a cell, alongside her daughter, who presumably will do so in more literal fashion. The punishment called back to Cersei’s own dark terror, expressed last season after Myrcella’s murder, about imagining dead loved ones decomposing in the grave. You will “watch her beautiful face collapse to bone,” Cersei told Ellaria.
Which is to say that like many punishments, it said more about the inflicter than the victim. It was a sign that even though Cersei 1) achieved ultimate power in spite of her enemies and oppressors, and 2) effectively solidified it through some savvy moves, she is still haunted by loss and fear and focused on settling old scores. At the same time Jaime, her closest companion, seems ever more uncertain about the future, which could be a problem.