The central theme of Roald Dahl's chilling short story "Lamb to the Slaughter" is that it is a mistake to underestimate those who appear to be weak, and that those who appear to be innocent are often anything but. While Patrick may deserve what he received, this story is about how people overlook the potential within Mary until it is too late.
The story "Lamb to the Slaughter" features underestimation on a number of levels. The first instance happens when Patrick comes home and tells Mary that he is leaving her. He knows that she is pregnant, but he just tells her that it is important to be calm and reasonable about a divorce, saying that he will send some money to help her take care of the child. When she tells him that he can't leave, he finally barks at her, "Try and stop me!" Mary clubs him to death with a frozen leg of lamb.
Patrick is a police detective, and when his colleagues arrive to investigate the crime, they don't even think of Mary as a suspect, even though statistics show that spouses are generally one of the more frequent guilty parties in killings.
Finally, as the hungry police officers devour the leg of lamb, which just happened to still be in the oven, they don't even think of it as the weapon, even though it would have been the perfect shape and texture (when frozen).