Dylan knew that Norman's father beat Norma, so Norman must have too unless it was much worse before he was too young to remember? But why would she wait so long to kill him if he beatings were consistent?
It's not angry beatings!
The father would tie Norma up and take a riding crop to her ass and shoulders.
Norman would secretly watch, and this is the inspiration for his torture porn manga.
But he took the game to far from game and too close to reality when he couldn't stop slapping her because it made his cock feel entirely really strong for once, and she responded to this by methodically arranging his murder a week later.
Cold as ice.