Mary Maloney certainly isn't Matilda's Miss Honey, but she's not really Miss Trunchbull, either. In fact, she's the model 1950s housewife. Dutiful even to a fault, she takes pleasure in providing her husband with a pleasant and relaxing atmosphere to come home to. She loves Patrick and his company and wants nothing more than to share a happy life together with him. However, when her unwavering and devoted service is thrown away for no apparent reason, something breaks in her brain.
With a child soon on the way, it would seem that Mary's life of wedded bliss is almost complete. But it's all been jeopardized by Patrick's sudden decision to end it. In total shock and denial, Mary is unable to make sense of this decision, and it would appear that she simply goes into survival mode. A pregnant widow would've had a much easier time in the '50s than a pregnant divorcee.
Although she is dazed and in shock right up until she whacks Patrick on the head, as soon as the deed is done, she is remarkably clear and calculating. As the wife of a detective, she knew what the punishment would be, so she hurries to cover her tracks, and does so quite successfully. Perhaps too pleased with herself, although some might find her actions justified, she even laughs when she overhears the detectives talking about their search for the weapon, which they happen to be presently consuming.