In "Lamb to the Slaughter" by Roald Dahl, how is motivation used?

I believe that this question is asking about character motivation and what motivates that particular character to act. Mary Maloney is a good character to use regarding motivation. When readers are first introduced to Mary, she exists to serve her husband. She is patiently waiting for him to get home, and once Patrick gets home, Mary flutters around the house getting him food and drink so that he can relax after his day of work.

She wasn’t really watching him, but she knew what he had done because she heard the ice cubes falling back against the bottom of the empty glass when he lowered his arm. He paused a moment, leaning forward in the chair, then he got up and went slowly over to fetch himself another.
“I’ll get it!” she cried, jumping up.

I assume that her motivation at this point is true love. Maybe complete infatuation, but the end result is the same. She is motivated to do whatever it takes to keep Patrick happy. When he announces that he is leaving her, that motivation ends. Soon after, Mary kills Patrick.
Then Mary's motivation shifts to protecting her unborn child. The text tells readers that she is completely willing to take on the punishment for her actions, but she is not willing to risk the life of her baby.

As the wife of a detective, she knew quite well what the penalty would be. That was fine. It made no difference to her. In fact, it would be a relief. On the other hand, what about the child? What were the laws about murderers with unborn children? Did they kill them both--mother and child? Or did they wait until the tenth month? What did they do?
Mary Maloney didn’t know. And she certainly wasn’t prepared to take a chance.

Protecting her child is Mary's motivation. It is what causes her to plan out an alibi for the murder and get away with her actions.