Who kills Alison in Pretty Little Liars?

Answer 1

Alison DeLaurentis had a twin sister named Courtney, although none of the Pretty Little Liars knew it. They traded places and names, so Aria, Hanna, Spencer and Emily befriended Courtney under the name Ali. The REAL Alison killed her sister Courtney, but since the Liars didn't know there was a twin, they thought Ali was dead. The truth is that the Alison that THEY knew was dead, although the real one was alive. So Alison's killer is.... herself. "A" is Mona, Hanna's wannabe follower. When Mona dies at homecoming (population change, anyone?) Alison takes her place sending messages to the Liars. Ali knew everything about Aria, Hanna, Emily and Spencer because Courtney told her everything
Nobody kills Allison, Allison is not dead until the last book. She has a twin named Courney and Courtney is crazy, so she went home to switch hospitals and she met the 4 girls. She pretended she was Allison and Allison went off to a hospital. Their friend Allison was actually Courtney, so Allison killed Courney in the seventh grade because Courntney ruined Alisons life. Alison thought it would be easy to convince her parents she really was Ali but instead they thought she was Courtney, and she went off to the hospital for 4 more years. And then Alison sets a fire in the house so the others die because they ruined her life, but they all make it out alive, and they think Ali dies in the fire but she could still be alive. Ali killed Ian and Jenna, and tries to kill Melissa.