Happy Death Day 1 and 2U Spoilers.
So, on the off-chance that there is a Happy Death Day 3 film, I figured that I should give my own working theory on the plot of what I suspect was always intended to be a trilogy. With the release of 2U and its introduction to parallel worlds, different lives and outcomes for the main cast in those parallel worlds, the parallel worlds causing different and even multiple Babyface murderers to exist, and Ryan traveling to the past to destroy the machine before vanishing out of existence; I think we can firmly say that this theory is entirely plausible. But why Carter Davis? Why would Tree’s boyfriend – even an alternate universe version – become the killer in the next movie? Well, it’s possible that the story writers have actually set it up in the beginning. Allow me to explain.
In Happy Death Day 1, there is a strange scene in the hospital where Tree wakes up in a delirious state and almost has a panic attack as she sees the Babyface killer coming towards her. At the time, she has no idea who the Babyface killer is. Once she hears Carter’s voice, she snaps out of her panic and can clearly see that it is Carter and not Babyface. Now, the killer in the beginning of the next movie is one of the main cast, Ryan – who is Carter’s roommate and presumably best friend – who is seemingly trying to kill Ryan or destroy the science project to prevent something horrible from happening. Let me just ask one simple thing: Why didn’t Ryan and his group just send Tree like throughout the entirety of the second film? The answer is likely because they couldn’t. Now, it is possible that Tree is simply too weak from dying repeatedly to risk this operation that the Babyface Ryan undertakes, but Babyface Ryan himself speaks of an apocalyptic future where he is the cause of something truly horrifying. It’s possible that these implications go far beyond the town and school in which Tree lives, but that simply gives Babyface Ryan more reason to do what he’s doing. Nevertheless, Babyface Ryan gets easily captured and stopped, so why didn’t a more experienced and resource savvy person like Tree go? It is possible that Tree’s worries of slowly causing her death to become permanent were foreshadowing for her eventual irreversible death in the next film.
There are a few things established in both films that lend plausibility to this theory besides what I just mentioned: Tree moving into parallel worlds did impact the relationship dynamic of Carter in relation to others as shown by Carter’s relationship to Danielle, characters change in personality in these parallel worlds like Lori who go from bloodthirsty killers to sensible and compassionate people, and likewise the Doctor from cheating buffoon to murderous Babyface killer. Moreover, Tree has learned to implicitly trust Ryan, Samar, Dre, and of course, Carter with information on parallel worlds, her ability to come back from death, and general information that she’s learned. If one of the inner circle of her friends was the killer, Tree probably wouldn’t be able to stop them until it was far too late as she probably wouldn’t be prepared for one among their group being Babyface despite knowing that some parts of their personality and relationship dynamics have changed. Carter would have enough information to lead her by the nose while possibly using her knowledge to proficiently kill whomever is Carter’s intended target. So for instance, Carter could trick Tree into believing that all possible enemies are dead or jailed, have Ryan close the loop, and then murder Tree once that was finished and Tree no longer poses a threat to his aims. Ryan would then try to dimension hop to kill his former self to prevent such a horrific outcome for his best friend and possibly Tree’s permanent death. Whatever the writers do from where, who knows. I suspect Ryan having pulled off some sort of miracle after vanishing in the beginning of the second film.
Nevertheless, it seems that Carter is being hinted as the Babyface killer at some point in the future in the first film and the parallel worlds of the second film were laying the groundwork for this plot twist in the third film.