Two Can Keep a Secret question

Maybe it was because Sadie broke up with Peter when they dated? I’m not too sure, but I guess that’s the closest thing that makes sense?

I literally finished the book that long ago but my first thought was that peter had got Sadie pregnant and he wanted to get rid of her because maybe he was looking into a more serious relationship with someone else, but I guess that idea gets shot down by the fact that Sadie lost her V card to Vance Puckett but then I thought maybe Sarah was Ellery's and Ezra's mother but it wouldn't of worked, I wish there was a very small page of information on why he wanted to kill Sadie but then again I like how it ended, idk I guess I liked cliff hangers but I kind of want to know why

Just finished the book and I think it is because Sadie was the homecoming queen right, she was the popular one and whatever so he wanted to kill her because they had a fling because he is in to young girls and then maybe he found out she was pregnant with another guys baby so he killed her but instead killed Sarah.

I think Peter was simply a bit twisted in the mind....
I mean obviously we know that both him and Sadie dated, maybe he was bitter about her breaking up with him.
But if you look closely all the targets were popular, well liked and in the Homecoming court... so maybe he has a grudge against that certain category of female.

So I just finished the book, and I read all of the theories but not all of them line up with the events in the book, so here’s my take on it. There is one main theory that I think makes sense, which is that Peter and Sadie did date and then she broke up with him or something went wrong and he being the sociopath he is accidentally killed her sister Sarah instead of Sadie, which isn’t a very far fetched theory because she then later on Sadie slept with another parent and got pregnant with Ellery and Ezra. This also fits his MO in which he targets young popular girls and then kills them once he’s done with them or they get in his way. We also don’t know about any other relationships he’s had so we can’t be entirely sure.