"Lamb to the Slaughter" is a snippet about the day when a happy and content pregnant wife receives news from her callous husband that he is leaving her. This not only changes Mary Maloney's life completely, but it also turns it upside down in a matter of minutes.
The reactions and posterior actions of the woman comprise the bulk of the story. These very actions are narrated from an objective and omniscient point of view that focuses on her state of mind before, during, and after her crime. While Roald Dahl seems aloof and unemotional throughout the narrative, certain details stand out, leading the reader to question---the way a juror would, should Mary be accused of Patrick's murder--whether Mary Maloney acted like a woman scorned, like an innocent wife who just snapped, or like a plain sociopath who even giggles as she realizes that she may never be "found out" for what she did.
[The weapon] is probably right under our noses. What do you think, Jack?
And in the other room, Mary Maloney began to laugh.
Since there is no definite answer to this question, an argument can indeed be made that the story touches on love, betrayal and self-defense, depending on the angle from which you analyze it.
If you focus on the lack of love between Mary and Patrick, then "Lamb to the Slaughter" would be the story of a broken-hearted, pregnant woman whose love for her husband leads her to commit an act of despair after learning from him that he no longer loves her.
You can support this thesis by noting how Mary dotes on her police officer husband on a daily basis, and how her love and admiration for him prevents her from noticing that something is wrong, that Patrick expects all of this devotion and hardly seems to appreciate it. He takes her for granted. The fact that Mary is heavily pregnant makes her case all the more tragic, and the love story takes a more morbid and pitiful tone.
Love can also be argued from the point of view of Mary as a future mother. The love that she bears for her unborn child leads her to snap and commit a crime. Therefore, it is arguable that she kills Patrick out of panic, that fear that all loving mothers might feel at the thought of raising a child fatherless and in misery.
As a story of betrayal "Lamb to the Slaughter" could be close-read from the point of view of police officer Patrick Maloney, a man who decides to tell his wife that he no longer loves her and no longer wants to be with her. Yet, his wife is not just loving and doting, but she is also the mother of his unborn child. That he coldly and carelessly tells her that he no longer cares is one thing; to literally leave her out of his life for good is quite another. This is betrayal at its worst.
The self-defense thesis involves what was said earlier about Mary as a future mother. The story is set in a time and place where women were sort of expected to be wonderful wives and mothers (1950s America). As such, Mary was already fulfilling the roles that society bestowed upon the women of her generation. That Patrick comes and takes it all away from her is not just odd, but cruel. What else could Mary do except cover her tracks after snapping and killing her husband? What would she do otherwise? Turn herself in, pregnant and with no prior criminal history, to serve time with career criminals?Hence, Mary is a woman who commits a crime of passion and needs to save herself, and her baby, from a cruel, male-dominated society.