First of all, he shocks us when Mary knocks her husband over the head, killing him. This is incredibly surprising, because she was such a docile, sweet, loving character; she didn't even react emotionally when her husband told her that he was leaving her. She just calmly went downstairs to get something for dinner. Then, out of nowhere, bam! She nails him on the head. That initial shock to the reader really sets the stage for the suspense to come. We are jarred and left tense, wondering what she will do. Then, she is STILL calm, and we are thinking to ourselves, "Why isn't she freaking out right now? What is she planning?" That is very suspenseful.
When she goes upstairs and practices smiling and talking in front of the mirror, that creates suspense because we have no idea what she is planning. She is formulating some sort of scheme, but the author doesn't let us know what she is thinking, so we don't know what it is; this creates suspense. We are on the edge of our seats as she goes to the grocer, lies to his face, plays it cool; her calmness alone creates suspense.
The author creates the most suspense when he has the cops come right to her house. We are anxious that they will find out what has happened; we wait, tense, as the questions are asked, as they search the house. Then, the final dose of suspense; Mary has the audacity to feed the officers the lamb. They are entirely clueless, but we are worried the entire time that they will figure it out.
So, with an initial jarring shock at the murder, through leaving out her thoughts at key points, and through bringing the cops to the scene (and weapon) of the crime, Dahl creates suspense for the readers.
Actually, the reader is taken by surprise much as Patrick Maloney himself! There is no time to wonder how the murder will be committed since no one suspects anything. However, the reader is aware that Mrs Maloney feels betrayed and totally let down, and especially vulnerable given her maternal state. For this reason, her sudden brutal act can be justified, even if it is "out of character" with her character profile.
There is suspense, however, after the murder scene in that the reader follows Mrs Maloney around in her meticulous steps to cover her crime. One can wonder how such a sentimental and docile wife can show herself to be so calculating and cool-headed. When the police arrive, the reader waits for the moment which never comes - the moment Mrs Maloney gets caught. There is an unfulfilled expectation.
Mrs Maloney's giggle at the end also takes the reader off guard. Not exactly the grieving widow, perhaps she is even enjoying " getting away with murder."