Desire has been roused in him. This beautiful creature has released in him a lifetime of repressed desire, hidden until now, like Janet Leigh's breasts have been hidden, beneath the bra, beneath the sweater, and he doesn't want to look at them. But he does want to look at them. That's why he's standing at the peephole .
As long as he's the one doing the looking he's fine. But if Janet Leigh would look up... Then she would become human and Tony isn't used to dealing with humans. He prefers objects, and what he's doing now is making her into an object. As is Alfred Hitchcock. In the movie, when we see Janet Leigh taking off her clothes we don't see Tony. Instead, we become Tony, and Janet Leigh becomes another object.
She's standing right there in front of him, and without meaning to she's teasing and taunting, and the boy (who lives with his mother) is pulled toward feelings he cannot have. He has them, but he cannot have them and therefore something has to change, and the thing that changes is the boy or man himself. The injunction is there; he can not not not experience desire, because it goes against the wishes of his mother, his dead mother, whose voice is clear and strong and vindictive, and so what he does is deny the person who has the desire. Himself. When he feels desire coming up, or anything, inside of him coming up, he automatically denies its existence. He denies it because he hates it; he hates it because it's painful; it's painful because it frightens him; it frightens him because it's forbidden; it's forbidden because he remembers his mother, and it threatens her. And when something threatens her...
He doesn't want to think about that. He's spent a lifetime not thinking about that or feeling that, and as he watches Janet Leigh the feelings she inspires in him and the struggle she creates in him become unbearable. That's when he runs out of the parlor and up the steps to the old house. We see him hesitate at the foot of the staircase, then walk down the hallway to the kitchen, where he sits in his chair at the table, rocking and listening. Listening to his own voice which is silent, and his mother's voice which he hates, deciding what to do.