MARION: Am I acting as if there's something wrong?
HIGHWAY PATROL: Frankly, yes.
The question, "Is something wrong?" might mean, "Are you in trouble?: But the patrolman is also asking if there's something wrong with Marion—is she a bad person, or has she done something wrong? This is a film in which bad things are done by bad people. Crime isn't just something you commit; it's something you are. So the police officer is trying to determine not just what Marion did but who she is. Which, understandably, creeps Marion out.
LOWERY: After all, Cassidy, I told you— all that cash! I'm not taking the responsibility! Oh, for heaven's sake! A girl works for you for ten years, you trust her! All right. Yes. You better come over.
In the film, it's not clear whether this conversation is taking place, or whether Marion is just imagining it. Either way, she knows that Lowery trusted her; he knows she's not a criminal. But then she took the money. So does that mean he's at fault for trusting her? Or is Cassidy at fault for waving all that money around? Guilt in Psycho spreads uncomfortably—so that Norman's mother is blamed for Norman's murders, and to some degree Marion is blamed for her own death. Alfred Hitchcock: not necessarily the dude to look to for moral guidance.
ARBOGAST: We're always quickest to doubt people who have a record for being honest.
Arbogast is trying to comfort Lila, who immediately believed in her sister's guilt. Still, it's an odd thing to say. Do you actually distrust people who have a record of being honest? Or is this just a reflection of the film's paranoid conviction that bad people are out there, hiding in plain sight?
NORMAN: I think I must have one of those face you can't help believing.
Again, innocence is presented as a sign of criminality. Norman's a sweet everyman. But under each of those sweet everymen is a knife-wielding monster. (According to that sweet everyman, Mr. Hitchcock.)
SHERIFF CHAMBERS: Your detective told you he couldn't come right back because he was goin' to question Norman Bates' mother. Right?
SHERIFF CHAMBERS: Norman Bates' mother has been dead and buried in Greenlawn Cemetery for the past ten years!
ELIZA: I helped Norman pick out the dress she was buried in. Periwinkle blue.
SHERIFF CHAMBERS: 'Tain't only local history, Sam. It's the only case of murder and suicide on Fairvale ledgers.
Fairvale is (as the name says) a nice place; they don't have murders there. Criminals go elsewhere. Except, as it turns out, that's not quite true. Norman has been murdering people for years; they're just mostly out-of-towners passing through so no one notices. Like Norman and Marion, Fairvale looks nice… but there are some sneaky criminals underneath.