What are some patterns that occur in "Lamb to the Slaughter" by Roald Dahl?
I am not entirely sure what you mean by "patterns," but certainly one re-occurring aspect of this tale is the use of ironic twists that occur throughout the tale and keep us guessing and laughing with black humour until the very end.
The first element of irony you will want to note and which can then be followed through in the tale is the actual title itself. This title is a deliberate allusion to the innocent, white lamb that was slaughtered. Clearly, Mary Maloney can be seen as this lamb at first, and yet, with situational irony, we see that it is she who kills her poor unsuspecting husband rather than the opposite. This irony is furthered as she, completely surprisingly for an unsuspecting housewife, displays massive criminal intelligence in organising an alibi for herself and then getting the police officers to ensure that the murder weapon will never be found, guaranteeing her freedom:
One of them belched.
"Personally, i think it's right here on the premises."
"Probably right under our very noses. What you think, Jack?"
And in the other room, Mary Maloney began to giggle.
Thus focussing on the various patterns of irony as they run through the story reveal the grimly humorous nature of this excellent short story.