Her punishment should have been
My interpertation of the ending was that Rachel in her her eyes was the embodiment of happiness idylic family, perfect love life, prefect body, and talented so when she killed her it was like she killed the concept of happiness and this is only reinforced by sending her to prison which is a breeding ground for misery. In prison she finally feels normal because everyone is just as messed up as her. So IMO her punishment should have been to send her back to HIGH SCHOOL.
share