Arriving at the Grandfather's house, Franklin examines the blood symbol on the van with a dried up "sausage" in his mouth. As the plot unravels, its obvious on 2nd viewing that Franklin has a dead man's penis in his mouth. Gunnar Hansen and Daniel Pearl point this out on the DVD commentary and the shy director Tobe Hooper laughs it off unconvincingly.
The iconic shot of Pam's red hot pants is gorgeous and ominous as this beautiful girl is slowly lured into a death trap like a fly in a spider's web. But on second viewing, you realize that this was a hypnotic camera effect that highlights her exposed back, which will soon have a rusty meathook rip through it.
The Hitchhiker placed a curse on the teenagers. The film is about the metaphysics of karma and the Law of Attraction (staples of the psychedelic hippie movement) as Pam explains the dark forces upon them with her astrology book and later Sally reveals that she thinks "everything means something, I guess". The Hitchhiker takes a photo of the teens and burns it when they embarrass him, thus dooming them. He cuts his own hand with Franklin's knife and then cuts Franklin's arm with his own razor, creating a blood ritual. When kicked out of the kids' van, the Hitchhiker smears a symbol in blood on their van, which Sally and Franklin notice later but no one else takes seriously. The ejected Hitchhiker does a psychotic ritual, blowing a raspberry, and later Franklin (as he too descends into darkness) shows the same strange behavior. The film is full of occult imagery and references to sunspots, probably added by writer Kim Henkel whose Texas Chainsaw Massacre: The Next Generation sequel is full of occult and conspiracy references. Director Tobe Hooper's own sequel The Texas Chainsaw Massacre 2 expounds on the major theme of "they live on fear... they live on fear".
Franklin is the carrier of the dark fear that kills the kids. His obsession with the slaughterhouse, the mutilation of cattle, the ridiculing of the Hitchhiker's family and giving directions to his friends, is what kills all of them. Only Sally survives because, as possessed as Franklin is, he unconsciously sacrifices himself for her, by DEMANDING that she take him with her even when she pleads that she can't "push him up that hill". Does Sally know that Franklin will die on a subconscious level? Its her survival instincts and disregard for her dying brother that actually saves her from being killed too. The filmmakers were an extremely cynical and hippie-hating lot.
This is further reinforced simply by Franklin's character design. He symbolically represents the entire Sawyer family. He has a knife and a fondness for the slaughterhouse (like Nubbins, the Hitchhiker). He's stocky with curly brown hair (like Leatherface in at least two of his masks), his clothes are even similar in design to Leatherface, as well as being all one color (like Drayton, The Cook). Lastly, he's wheelchair bound (like Grandpa Sawyer).
As the Hitchhiker is the only family member that full on embraces psychopathic, ghoulish behavior and we see that he plays with corpses for fun, Leatherface's masks are probably gifts from his brother and not actually made by the big infantile man himself.
Franklin calls the Hitchhiker's family a "a whole family of Draculas" and, in the end, a corpse-looking Grandpa sucks Sally's blood to revive himself.