The leg of lamb is an effective weapon because of its weight and structure. The lamb is shaped like a club—it has a thickened section, which is frozen solid and is therefore just as dangerous as a steel club. The other end is much thinner and therefore easy to grasp, much like the handle of a club. Because of this, it was easy for Mary to swing when she hit Patrick behind the head. The resultant force of the blow caused maximum damage and Patrick instantly died of blunt force trauma.
Roald Dahl not only chose the leg of lamb as the logical choice for the above reason, but also because of its symbolism. Lambs are associated with peace and harmony. They are gentle and meek creatures. When one thinks of a lamb, it generates images of warmth, something woolly and cuddly. One thinks of green pastures with flocks grazing idly. Surely one cannot contemplate that it could be a source of danger?
So it is with Mary. The lamb symbolizes her docile, almost servile, attitude. She presents as a harmless being, intent on providing only the best for her husband. She goes out of her way to please him and provide his every whim. Before she kills Patrick, the reader is surprised by her absolute admiration and appreciation of him and her insistence to please him:
She took his coat and hung it in the closer. Then she walked over and made the drinks, a strongish one for him, a weak one for herself;
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 intent, far look in his eyes when they rested in her, the funny shape of the mouth, and especially the way he remained silent about his tiredness, sitting still with himself until the whiskey had taken some of it away.
"I'll get it!" she cried, jumping up.
"Darling, shall I get your slippers?" and so on.
It is therefore ironically shocking later when this docile woman commits such a heinous act and cleverly covers up her misdeed by not only creating an alibi, but then also allowing the investigating officers to ingest the primary evidence, which has now been cooked and is presented to them as a meal. They are, of course, famished and merrily dig into the scrumptious offering, not realizing that they are getting rid of the evidence.