Like Norman, our gal Marion seems to be split between the Dark Side and the Light Side of the Force. She's a good girl gone bad. She's a bad girl with a heart of gold. Like the color palette of Psycho, Marion seems to be both black and white.
Psycho broke the mold in a lot of ways, but perhaps the biggest shocker for the movie going public in the early 1960's was that the protagonist of Psycho is killed off halfway through the movie. Yup: Marion Crane doesn't make it to the credits.
The next few thousand ways in which Psycho traumatized the audiences of 1960 have to do with Norman, Norman's mom, and Norman's fondness for butcher knives. But somewhere down the list was this: it was deeply shocking for people to see that their main character wasn't a good person.
Today, it's no biggie if a female character has sex with a man that she's not married to. It's no biggie (unless she gets fired) if she's doing that particular deed while playing hooky from her job. But in 1960 that nonsense was pearl clutch-inducing.
To top it all off, Marion steals. This is her most important action in the film; it's what most of her screen time is about. She takes $40,000 (which is a huge chunk of change in today's dollars) in order to help her boyfriend get on his feet financially. That's not a heroic act. Bad Marion.
So Marion does bad things. But the film assures us repeatedly that she's not really a bad person. Yes, she has sex with her married boyfriend, but she knows it's wrong, and she's resolved to stop. "Sam. This is the last time," she declares after their afternoon tryst at the beginning of the film.
She goes on to say that they're going to continue their relationship in another manner.
MARION: 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!
Marion is violating sexual conventions, but she's determined to stop, and behave like a "good girl" circa 1960.
Marion's guilty about stealing the money, too. She's no hardened criminal; she practically gives herself away every time she encounters anyone. ("Am I acting as if something's wrong?" she asks a cop who stops her on the highway. To which he responds "Frankly, yes.")
Check out what she says to Norman over dinner:
NORMAN: We all go a little mad sometimes.
MARION: Yes, and just one time can be enough.
In stealing the money—in having sex outside of marriage—she went a little mad. Marion acted uncharacteristically, passionately, and without due consideration. But deep down, she's morally sounds. She doesn't deserve to die.
It's the #1 rule in Scream: sex = death. And we know from the first moments of Psycho that Marion is doing the nasty outside of wedlock.
Yup: this film first makes Marion sexy… and then punishes her for being sexy. It punishes her with a knife. It punishes her in the shower. It punishes her to death.
You first see Marion in her underwear. Throughout the film, she's repeatedly sexualized, and presented as attractive and desirable. Cassidy, a rich real estate client, finds her attractive… and he doesn't keep it subtle, either. Secretary Caroline says to Marion:
CAROLINE: He was flirting with you. I guess he noticed my wedding ring.
This underlines another shocking thing about Marion: not only is she sexually desirable, she's sexually available. She's neither an innocent maiden nor a sexy-but-taken housewife. Gasp!
Norman Bates is—in the words of the psychiatrist who diagnoses Norman's psychosis at the end of the film—"aroused" by Marion. Norman puts Marion in Room #1, where he can watch her through his creeptastic hole in the wall. Unfortunately, being sexy around Norman is just about the worst idea. To paraphrase The Hulk: don't make Norman Bates horny. You wouldn't like him when he's horny.
The most famous scene in the film is the shower scene, in which Marion appears to be nude. In other words, the most vivid image of Marion is not of her being good, or bad; it's of her taking a shower—and of her being watched in the shower.
For Norman, Marion isn't really a person; she's a sexual fetish in his own psychodrama. And the film in many ways takes Norman's perspective. Marion is most memorable, and most remembered, as a sexual provocation. And that sexual provocation leads to—yup, you got it—being killed.
(Freudian aside: it's notable that Norman uses a knife to kill Marion—that he uses a long, sharp phallic symbol to, um, "get inside" her. This symbolism is diluted by the fact that he uses the same knife on Arbogast, though.)
Ultimately, the question of whether Marion is good or bad doesn't matter. It's an elaborate distraction. Psycho isn't a film about Marion as a person—it's about Marion as a sex object, and how Marion's sex appeal triggers a massive detonation inside Norman's skull.