the jealous mother
Norman Bates: [voice-over] Mother, please...!
Norman Bates: [voice-over] Mother, she's just a stranger. She's hungry, and it's raining out!
Norma Bates: [voice-over] "Mother, she's just a stranger"! As if men don't desire strangers! As if... ohh, I refuse to speak of disgusting things, because they disgust me! You understand, boy? Go on, go tell her she'll not be appeasing her ugly appetite with MY food... or my son! Or do I have tell her because you don't have the guts! Huh, boy? You have the guts, boy?
Norman Bates: [voice-over] Shut up! Shut up!
[Points finger at Lila Crane]
Dr. Fred Richmond: When he met your sister, he was touched by her... aroused by her. He wanted her. That set off the 'jealous mother' and 'mother killed the girl'! Now after the murder, Norman returned as if from a deep sleep. And like a dutiful son, covered up all traces of the crime he was convinced his mother had committed!
So it is confirmed that Marion's murder was indeed a murder for love.