Why is Mary Maloney's transformation surprising?

Mary Maloney proves herself a very round and dynamic character. That, in and of itself, shouldn't be shocking or surprising. Protagonists should be characters that show some depth to them and are changed because of the events that happen to them over the course of a given plot. With all of that said, I think Mary's transformation is surprising for two reasons. First, she is introduced to readers as a very stock/static character. Second, her transformation is so unbelievably drastic in a ridiculously short amount of time.
When readers are introduced to Mary, she is characterized as a quintessential doting housewife. She practically worships the ground that her husband walks on. She patiently waits for him to get home, she pours him his drinks, and we are told that her favorite part of the day is luxuriating in his very presence. If I'm honest, it's a bit gag worthy. Then Mary becomes a completely different person within moments of receiving Patrick's terrible news. She moves without thinking and clubs him over the head with a leg of lamb so hard that she kills him. You could argue that she acted without thinking and perhaps forgive her; however, everything she does after realizing Patrick is dead is incredibly cunning and manipulative. She figures out how to establish an alibi, she practices it, and executes the plan. Mary then figures out a creative way to permanently dispose of the murder weapon. Then the story ends by giving readers the final piece of information that shows that Mary is no longer an innocent, doting housewife just trying to protect her unborn baby. She giggles at what the police are talking about. She finds her husband's death and her getting away with the killing humorous. That's a magnificent transformation from the Mary Maloney we were introduced to at the story's start.

Perhaps the most prominent theme in Roald Dahl's story "Lamb to the Slaughter" has to do with the sudden and surprising change that comes over Mary Maloney when her husband tells her he wants a divorce. She was herself literally a "lamb" up to that point, but she changes into a murderess and kills her husband with one blow in a violent outburst of rage. The theme has been a common saying for centuries.
Hell hath no fury like a woman scorned.
It was originally derived from William Congreve's play The Mourning Bride (1697), in which the character Zara says:
Heaven has no rage like love to hatred turned,
Nor hell a fury like a woman scorned. Act 3, Scene 7
A good example of love to hatred turned can also be found in Medea (431 B.C.) by the ancient Greek playwright Euripides (480-406 B.C.). But many examples can be observed in our everyday modern life, or read about in the newspapers.
The most striking thing about "Lamb to the Slaughter" is not that Mary Maloney gets away with her crime, nor that she has the investigating policemen eat up the murder weapon, but it is the totally surprising change in her meek, loving character which motivates her to kill her husband with a single, violent blow to the head with a frozen leg of lamb.