Norman is famously the one in Psycho who's got another person behind his eyes. But the first person you see listening to inner voices isn't Norman. It's Marion.
Several times as she's driving away with the cash, we see a close-up of Marion's face and then hear other voices — her boss, Lowery, her coworker, the client Cassidy that she robbed. Cassidy's imagined voice says:
CASSIDY: Well I ain't about to kiss off forty thousand dollars! I'll get it back and if any of it's missin' I'll replace it with her fine soft flesh
The voices in Marion's head are a way for Hitchcock to let you know what's going on with other people—or at least what might be going on with other people—without moving away from Marion's perspective. The first part of the film is determined to always stay in Marion's head, in the interest of making it all the more shocking when the film moves to somebody else's.
But there are other reasons to listen to the voices in Marion's head, too.
Philosopher Slavoj Zizek points out that the last time Marion hears these voices "when she listens to the imagined voices of her boss and the millionaire who bought the house, furious at her deception, her expression is no longer anguished." Instead, Zizek says, "what we perceive is a strange manic smile of a deeply perverse satisfaction, an expression which uncannily resembles the very last shot of Norman-mother, just before it dissolves into the skull and then the car appearing out of the swamp." (Source)
Both Marion and Norman hear voices in their heads; both seem to find these voices pleasurable in a twisted way. Norman and Marion are therefore linked; they're both guilty of crimes, and while both are on the surface disturbed by their crimes, they both actually take pleasure in them. They're both not in their right minds, so not guilty—but the fact that they take pleasure in not being in their right mind makes them culpable… and maybe a teensy bit deserving of their fates.