What made Norman Bates into the killer in Psycho? Well, that was already answered in the Showtime movie Psycho IV: The Beginning, but A&E is going to explore it again in a new series called The Bates Motel.
The new series will "offer an understanding into how Norman Bates' psyche developed and would tell the back story of the film's killer, learning of how his mother, Norma, and her lover damaged him, transforming him into serial-killing motel owner." Again, that understanding was previously given in Psycho IV.
To be fair the Psycho series is a little weird, continuity-wise. Psycho IV ignores many elements of the previous two movies, including Norman's aunt, Emma Spool, who may (or may not, depending on what movie you're watching) have been Norman's real mom. And there was already a Bates Motel TV pilot. That one starred Bud Cort as Norman's asylum cellmate who inherits the Motel after Norman's death (Psycho IV came out after this, and Norman's alive in that). The show never got picked up but the pilot aired as a TV movie.
I wonder how they're going to get a TV series out of Norman's mom being an asshole to him. It doesn't sound like fertile ground for a weekly. I'm curious as to how this turns out.