MARION: Oh, we can see each other. We can even have dinner but respectably in my house with my mother's picture on the mantel and my sister helping me broil a big steak for three.
SAM: And after the steak, do we send Sister to the movies? Turn mama's picture to the wall?
Norman's mother is worried about her son having sex; Sam is here joking that Marion's mother would also object to her daughter having sex. Mothers demand their children conform to decency—and that demand leads to indecency. Norman murders because he's afraid of illicit sexual thoughts. Marion steals because she wants to marry Sam, rather than continuing a sexual relationship with him that mother would disapprove of. Mothers: everything is their fault. (Or at least everything is there fault in Psycho. Maybe we should blame Hitchcock's mother.)
NORMAN: A boy's best friend is his mother.
This sounds sweet at first: aw, Norman loves his mother. But, really, you shouldn't love your mother that much, should you? In Hitchcock's time, men weren't supposed to be too attached to their mothers. Norman is a momma's boy—which means he's sexually stunted and ultimately a murdering psychopath.
NORMAN: You know what I think? I think that we're all in our private traps, clamped in them, and none of us can ever get out. We scratch and we claw, but only at the air, only at each other, and for all of it, we never budge an inch.
MARION: Sometimes... we deliberately step into those traps.
NORMAN: I was born into mine. I don't mind it anymore.
MARION: Oh, but you should. You should mind it.
NORMAN: Oh, I do... [laughs] But I say I don't.
Norman was born into his trap—which is to say, he was born into his relationship with his family, and particularly his mother. The family in Psycho is a swamp that you get stuck in—though really, you could say Norman's mom is the one that got trapped. He killed her, not the other way around.
NORMAN: Mother! Oh, God, Mother! Blood! Blood!
Motherhood is frequently associated with blood and menstruation. Norman's shout is about Marion's death, but it could also be seen as a terrified fear of his mother's sexuality or fertility. Sex, blood, mothers—they're all banging around in Norman's fractured psyche. You could say Norman was driven insane by the thought that his mother had to have sex to give birth to him.
NORMAN: [voiceover in police custody, as Norman is thinking] It's sad, when a mother has to speak the words that condemn her own son. But I couldn't allow them to believe that I would commit murder.
This is Norman, thinking as his mother. Norman/mother claims that Norman/son is the real murderer. But it was when Norman/son thought he was Norman/mother that he committed the murder. Who is really guilty? It doesn't exactly matter, because the point of Psycho (and of Freud) is that mother lodges in the child's psyche; the child is partly the mother. So the mother is always responsible for what the child does. Which hardly seems fair given that mom here is a corpse, but Freud and Hitchcock aren't known for being fair, especially when women are involved.