After Mary Maloney realizes that she has killed her husband, her reaction is very calm, considering the circumtances. One might expect her to freak out, to go into hysterics, to weep and sob, to have a panic attack, to pace about, etc. Instead, she calmly assesses the situation. Dahl writes of her reaction,
"All right, she told herself. So I’ve killed him. It was extraordinary, now, how clear her mind became all of a sudden... What were the laws about murderers with unborn children?...Mary Maloney didn’t know. And she certainly wasn’t prepared to take a chance."
As she realizes what she has done, she also realizes that her baby might be in danger. She knew that she needed to cover up the crime to protect the child. So, being a detective's wife, she realizes that she needs an alibi. Dahl points out that she is a detective's wife, and that probably aids in her ability to remain calm and to assesss the situation more clearly in terms of what needed to be done. She goes to the store, concocts a story about Patrick being tired so not wanting to go out. This establishes an alibit--she was at the store, so couldn't have murdered him. Also, it leaks information that Patrick is at home, alone--the perfect setup for a murderer to enter. She even mentions the lamb in the oven, to cover any possible suspicions of it being the weapon. Then, on the way home, she tries to psych herself out to get in the right mindframe for making the call to the police. She needs to sound shocked; on the way home she thinks,
"And now, she told herself as she hurried back, all she was doing now, she was returning home to her husband...and if, when she entered the house, she happened to find anything unusual, or tragic, or terrible, then naturally it would be a shock and she’d become frantic with grief and horror. "
This little pep talk works, and as she walks in and sees him on the floor, she actually feels shock. She has been so calm this entire time, and seeing him there really hits her. She uses this emotion in the phone call to sound sincerely upset. After that, it isn't hard--the house is swarming with policemen, asking her questions, and they take the lead. The last bit of cunning on her part is getting them to eat the evidence; very clever, and it isn't until the cops mention that the weapon is probably "right under our very noses" that she loses it and begins to giggle. The story ends there, so who knows if she got a grip or lost it completely, but at least through the story, she displays remarkable calm and cleverness. I hope that those thoughts helped; good luck!