How are "actus reus" and "mens rea" connected or linked to the story "Lamb to the Slaughter?"
Most literally, actus reus means "guilty act" and mens rea means "guilty mind". Connected, the two mean a crime has been committed. If a person sets out to steal your stereo, they have mens rea; when they actually do, they have committed actus rea. If someone mistakenly takes your stereo because they honestly believe that it is theirs, then there is no mens rea and no crime (provided they return the stereo when the mix-up is revealed).
In this story, there is clearly a guilty act (actus reus). Mary walks up to her husband and hits him with enough force to kill him. It is no accident. However, readers often disagree about the presence of mens rea, suggesting that because Mary is in shock over her husband's betrayal, she is not conscious of her act. This can be reinforced by her honest tears of grief when she returns from the grocery store to see her husband's corpse.
Others, however, suggest that Mary's amusement throughout the ordeal suggest vengenance, meaning she does have a guilty mind and was consciously trying to get revenge on her husband for saying he's going to leave. For example, she actually stops to consider dessert choices just after Patrick's death. And then, when the police speculate about the murder weapon, she actually laughs:
And in the other room, Mary Maloney began to giggle.
This could be suggestive of her pleasure in her husband's death.