I have a fat theory. First starting with this. (I got the second pic from Plliarsanonymous.tumblr.com so thank you)
I’m pretty sure the girl that Spencer hit that night is the girl Sara. I think Sara’s body is in Ali’s grave. But I don’t think spencer killed her.. We see blood spatter on her face but maybe she knocked her unconscious or something. I just don’t wanna believe spencer killed someone. This is all from Spencer’s memory so maybe parts are not 100% accurate. They’re still coming to her.
Spencer asks Mrs. D something like “if you saw me hit Ali why didn’t you call the cops?” And I feel like I know why.. I honestly think Mrs. D didn’t want Ali alive. So she saw spencer try to kill Ali that night and didn’t stop her. Then she realizes that the girl spencer was chasing wasn’t Ali, it was Sara. So she buries Ali alive in the backyard and says Sara’s body is Ali’s and no one would ask any questions because Mrs. D thinks Ali is dead in the backyard! And maybe spencer knocked Sara unconscious, but Mrs. D finished the job so she could finish he plan of using Sara’s dead body as Ali’s. And I think since what happened that night is coming back to spencer, Spencer’s gonna remember what Mrs. D did, so she’s messing with spencer now trying to make spencer believe that she is the one who hit Ali that night, so Mrs. D is in the clear.
Ali probably saw Ezra stalking the girls so she thought that he was who she should be afraid of, but then she finds out he’s just researching and not actually trying to hurt anyone so that’s why she tells Emily “I thought I knew who I was afraid of but I was wrong.” Or something like that. Ali says she can’t come home and that’s because her mom, who wanted her dead, lives at her home! The girls are putting everything together like Ali said they would. They’re remembering things they didn’t before and holes are getting filled and it’s becoming clearer who they should stay away from.