In Pretty little liars -the TV show everyone thought that Alison's killer was Ian because he was one of the last few people to see her alive the night she disappeared and he had home videos of Alison and the PLLs when hey were younger and one from the night of The Jenna Thing. At one point spencer's older sister Melissa Hastings' hockey stick was buried in The Hasting's backyard and was found by Toby while doing construction work on the dilaurentis house.. Mr Hastings then later burned the hockey stick. but in the summer finale of season 2 it is revealed by detective wilden that a hockey stick didn't kill Alison but a shovel did... and he knew all along what the murder weapon was. but anyways i believe that Alison's killer is Aria because she never had a flashback from the night Alison disappeared and Ali never came to her in any hallucinations and aria was never personally attacked or had a near death experience with "A". And another reason why i think aria could be Alison's killer is because in the opening theme song when the girls are putting Alison in her coffin and putting make up on her aria puts her finger to her lip and then Shushes at the end of the video as if she has a bigger secret than the other girls .. and in the season 2 poster when the girls are wearing white party dresses under the table the shovel that killed Alison is underneath aria's foot and she is still shushing in the picture.. and that's my theory of Alison's killer in the TV show.