Talk:Run, Ali, Run/@comment-185.15.0.13-20140716122414/@comment-175.156.114.180-20140716171545

Actually Spencer already said the reason out loud in the show. In the video that Alison received from A in the cafeteria, A mentioned about burying Jessica the same way Jessica buried Alison, implying that A may have been the person who hit Alison in the head (and standing closeby when Jessica buried Alison), which also suggested A was the one who (killed Jessica and) buried Jessica. In season 2, Peter Hastings had more than once implied that he thought Spencer had killed Alison (who is actually Bethany). So when the liars concluded that the two murders were committed by A (Aria also rebutted unless Peter is A), Peter couldn't have killed Jessica if he wasn't the person who hit Alison and killed Bethany.

Person hit Alison & Person killed Bethany = Person killed Jessica = A Peter tot Spencer hit Alison =/= Peter hit Alison & Peter killed Bethany therefore Peter =/= Person kill Jessica =/= A

It's based on the rules of elimination, however this theory is ultimately very flawed as 1. Spencer believed her father when he said he thought Spencer was the one who hit Alison, which was why he covered up so much to protect Spencer 2. A mentioned watching Jessica buried Alison, but Bethany was the one being found. To sum it up, the liars have assumed that whoever hit Alison is the person who killed Bethany, which may not be true. There is a delay in between the two events that has yet to be mentioned - Alison didn't crawled out from the ground immediately otherwise her mum would have saw her, and Bethany was buried in replaced of Alison, but who buried her was undetermined.