At this point, we've talked about R, Julie, Nora, and M. We're almost out of Romeo and Juliet characters to compare these guys to. Who's left, who's left? Hmm... Oh. Right. This guy. We described Juliet's first love, Paris, as "a stiff [...] lacking in passion." That sums up Perry Kelvin right there.
He wasn't always that way, though. Julie says he "was this brilliant, fiery kid, so weird and funny and full of dreams" (1.7.56). Then he just gave up. Although Julie never words it this way, he was like a zombie—wandering around life with no purpose. How many people like that do you know?
Basically, Perry became Julie's dad. We've lost count of the times Perry says something along the lines of, "At least I can protect her. At least I can keep her safe" (1.2.41). Her dad is the same way, valuing security over, you know, living life. And R, with Perry's thoughts inside of him, starts thinking the same thing. What are they going to do, lock her up in a crate, regulate her food intake, and keep her from interacting with the outside world? Oh wait, that's exactly what they've done. The stadium they live in is just a giant crate turning all of humanity into under-developed veal. Tasty for zombies.
If it weren't for Perry's thoughts, R would have killed and eaten Julie, too. But Perry's thoughts are dangerous, and R doesn't quite realize that fact. All that "safety" talk, that's what got humanity into this mess in the first place.