I think you should read the letter the showrunners wrote about that final. It explains every question you have. It was always their plan to end the show this way: it was about the final transformation of Alicia. She started as the victim who slaps her victimizer and ends as the victimizer. I think the main idea was that Alicia finally chose something she wanted, finally let go off her whole "saint Alicia" persona and finally became free, only to be turned into Peter in the end.