For the characters of The Sound and the Fury, sin is almost always related to sex. In fact, at times the two are synonymous. A daughter’s blossoming sexuality becomes the tipping point which throws an entire family into chaos. Even though sex is at the center of the novel, it’s something that the characters can’t address fully. If they admit that someone has moved beyond the morals preached by older generations, then they’re left navigating unknown territory – something which moves one brother to madness and another to all-consuming rage. Sin, we learn, is a relative term – one that changes according to the shifting perspectives of individual characters.
Caddy’s sin isn’t that she’s sexually promiscuous – it’s that her promiscuity undercuts her ability to behave as a "lady."
Because he’s not mired in notions of morality or right and wrong, Benjy Compson remains the one character able to articulate his relationship with Caddy in terms of love.