I think the author Roald Dahl makes it clear that Patrick Maloney is not leaving his wife Mary because he is involved with another woman. He is so regular in his habits that he could hardly be seeing another woman on the side. This is good for Mary because it eliminates one possible motive she might have for killing her husband. The police could find out pretty easily if Patrick had been having an affair.
The author shows in dramatic fashion why Patrick wants out of the marriage. He feels suffocated by the claustrophobic domesticity he comes home to. He has no privacy. They sleep in the same bed. His wife is too loving, too mothering, too suffocating, too needy, too clinging. As a matter of fact, John Collier explains very clearly what is wrong with most marriages in his story "The Chaser." The old man who sells love potions and "chasers," or antidotes, tells his impetuous, lovesick young client Alan Austen:
"You will be her sole interest in life....She will want to know all you do....All that has happened to you during the day. Every word of it. She will want to know what you are thinking about, why you smile suddenly, why you are looking sad....How carefully she will look after you! She will never allow you to be tired, to sit in a draught, to neglect your food. If you are an hour late, she will be terrified. She will think you are killed, or that some siren has caught you."
The old man feels sure that his customer will someday want to kill his wife. The wife he describes to Alan Austen sounds very much like Mary Maloney. No doubt, Patrick already feels like killing her. She doesn't understand that her attentiveness is driving him away from her. Most people need privacy some of the time. As the Prophet says in Kahlil Gibran's well-known book The Prophet:
But let there be spaces in your togetherness, And let the winds of the heavens dance between you. Love one another but make not a bond of love: Let it rather be a moving sea between the shores of your souls.
The fact that Mary's loving devotion can turn so suddenly into murderous rage seems to indicate that there must have been some hatred hidden underneath all her slavish attentiveness. She doesn't change into a different person. Nobody can do that. She is the same Mary Maloney but she reveals feelings and cunning nobody, including herself, ever knew existed.