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.