There are at least two ways to tell that Mary loved her husband up to the time he told her he was leaving. For one thing, the author Roald Dahl devotes a long early paragraph to describing how much she loved Patrick and for what reasons. Dahl wants to highlight his theme that love can turn to hatred, which is what the story is really all about.
For her, this was always a blissful time of day....She loved to luxuriate in the presence of this man, and to feel--almost as a sunbather feels the sun--that warm male glow that came out of him to her when they were alone together. She loved him for the way he sat loosely in a chair, for the way he came in a door, or moved slowly across the room with long strides. She loved the intent, far look in his eyes when they rested on her, the funny shape of the mouth, and especially the way he remained silent about his tiredness....
So Dahl tells us how Mary thinks and feels. And then the author shows how much she adores her husband by describing the slavish way in which she dotes on him. For example:
"Darling," she said. "Would you like me to get you some cheese? I haven't made any supper because it's Thursday."
"If you're too tired to eat out," she went on, "it's still not too late. There's plenty of meat and stuff in the freezer, and you can have it right here and not even move out of the chair."
Her eyes waited on him for an answer, a smile, a little nod, but he made no sign.
"Anyway," she went on, "I'll get you some cheese and crackers first."
She moved uneasily in her chair, the large eyes still watching his face. "But you must eat! I'll fix it anyway, and then you can have it or not, as you like."
Dahl never tells the reader exactly why Patrick is tired of his wife and why he wants out of their marriage. But it should be abundantly clear to the reader that she is smothering him with her affection, attention, and devotion. We can see that he is not the type of man who likes such endless and monotonous domestic felicity. He is a tough cop. He is a man of action. That is obvious. He has put up with Mary's "mothering" for as long as he can stand it. But now that she is going to have a baby, he can see that his imprisonment in their claustrophobic little house is going to get even harder to bear.
Ironically, it is Mary's too-strong adoration that is the cause of Patrick's discontent. And it is her overpowering need for his love and constant attention that finally leads to his death. This is one example of what can go wrong in a marriage. The problem with love is beautifully expressed by Claudius in Shakespeare's Hamlet.
There lives within the very flame of love
A kind of wick or snuff that will abate it. (4.7)
Patrick must have loved Mary--or thought he loved her--at one time. But now his love is very obviously dead. And it is dead at a time when it ought to be even stronger. She is going to have his baby. It is a further irony that Mary's love for Patrick dies at the moment she realizes he no longer loves her. This "epiphany" occurs at a time when she happens to be holding a frozen leg of lamb.
What is "love," anyway? Many popular songs are cranked out every year about how much somebody loves somebody, and many songs are written about how somebody is mourning the loss of a loving relationship. The former are called love songs; the latter are called torch songs. One of the oldies is titled "What Is This Thing Called Love?"