How does Dahl use contrast?

Roald Dahl uses one of the oldest literary and artistic devices in "Lamb to the Slaughter." This is the device of contrast, which is to be found in almost any good work of art. When the story opens, Mary is waiting for her husband to return from his job. She is six months pregnant, and this condition makes her feel happy and peaceful. Here is Roald Dahl's description of her mental and emotional state:
Now and again she would glance up at the clock, but without anxiety, merely to please herself with the thought that each minute gone by made it nearer the time when he would come. There was a slow smiling air about her, and about everything she did. The drop of a head as she bent over her sewing was curiously tranquil. Her skin—for this was her sixth month with child—had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger and darker than before.
Mary loves her husband, and she believes he loves her. She is looking forward to having their baby, and she believes he is looking forward to it, too. But her husband Patrick is in an entirely different mood. He has been brooding over his feelings about their relationship for a long time, and now he is ready to drop the bomb.
Mary's change of mood from a loving wife and blissfully expectant mother to an enraged murderess is all the more understandable—and effective—because her mood had been so utterly beatific before Patrick's announcement. It was this sudden betrayal and denial of everything she wanted and treasured that shocked her out of her dreamworld into the world of reality and prompted her to take such a drastic action. We can believe in Mary's transformation because we identified and sympathized with her when she was still in a state of blissful ignorance.