Who killed alison delaurentis?

No one, she isn't dead. Ali had a twin sister named Courtney, and Courtney wanted to be her. So when Courtney was being transferred to another mental facility, she switched places and Ali's parents sent Ali to the facility by mistake. She tried telling her that she was Alison, but she didn't buy it because Courtney pretends to be Ali. So the Ali that the four girls knew was Courtney. So that night when Spencer gets into a fight with Ali(Courtney) Courtney gets killed. Ali killed Courtney out of revenge for ruining her perfect life. Thus Ali killed Ali.