What is the main idea of "Lamb to the Slaughter?"

The phrase "A lamb to the slaughter" refers to lambs who
are slaughtered for their meat. The also used to be slaughtered so their skins could be
made into particularly good gloves.

The lamb is young,
ignorant, and weak.  If he is treated well he will not fear people; if he feels
threatened he will likely freeze -- lie down and hold still so as to escape detection. 
So it is very easy to slaughter a lamb.  A lamb has no weapons, so he can't possibly
hurt you.

Mary Maloney starts off as a lamb: she is in a
warm, comfortable, loving environment; all her needs are met; and she is secure and
content.

Then her husband threatens to destroy her ideal
world.  Suddenly the situation is turned upside down: the former lamb becomes a very
efficient predator.  To preserve her pleasant life she kills her husband, without
malice, anger, or even regret -- and, by the author's sense of humor, with a frozen leg
of lamb. She very efficiently conceals her tracks, again not allowing emotion to
interfere with her actions.  Again by the author's humor she causes the police to eat
the murder weapon; of course the last thing they say in the story is that the muder
weapon is probably right under their noses, and they are quite
correct.

I think the main idea is that even a lamb like
Mrs. Maloney can turn deadly when threatened.  Note that the Mrs. Maloney at the start
of the story is very different from the lady at the end.  She is now capable of most
anything, and it's hard to predict how her life will go in the
future.