Amazon's I Know What You Did Last Summer features some brutal kills — but in terms of suspects for who is doing those slayings, it's really anybody's guess. In fact, many of the potential suspects (like, say, creepy convenience store worker Dale) have met a bloody fate themselves. That may mean that the killer is one of the people we've come to trust — and if that is the case, I have my eye on social media superstar, Margot.
Margot might seem like an unlikely suspect, but that may be the very reason she makes the most sense. In fact, if you look back on each episode, Margot as the killer just kind of works — especially when you consider her motives for torturing her friends.
Margot was in the car when Allison hit her twin sister Lennon and killed her. In fact, Margot was the driving force behind making sure no one went to the police, and she actively encouraged the friends to cover up the crime and dump the body — she didn't want anything jeopardizing her future. Except, at the time, Margot thought it was Allison, Lennon's sister, who was killed in the accident. What Margot didn't know at the time is that Allison was posing as Lennon that night, as she has been ever since.
This, to me, is the strongest piece of evidence that Margot could be the killer, and that the "I know what you did last summer" message isn't just about killing Lennon, but about Allison using it as an opportunity to become Lennon. Margot was crazy-in-love with Lennon, and confessed as much to her on graduation night. That's why, I think, Margot likely learned the truth about the twin switch at some point during her year away — and that this fueled her desire to punish Allison.