What literary devices does Roald Dahl use in "Lamb to the Slaughter"?

The
first technique Dahl employs is contrast. He
juxtaposes Mary Maloney's actions and words with those of her
husband. The contrast is introduced when Mary pours him a strong drink and a weak one for her.
This in itself symbolizes the difference at this point between the two. When Mary, obviously a
doting and caring spouse, speaks to her husband, she uses a caring tone. She is practically
servile in her approach and is insistent that she wants to do good. Her husband, on the other
hand, displays a brusque, off-hand manner marked by short, terse expressions. It is evident that
he has something on his mind and one expects that he is about to tell his docile, loving wife
exactly what it is.

In this regard, Dahl also
foreshadows what is to come. It is easy to ascertain from Patrick
Maloney's manner that he has nothing good to share with Mary. Dahl creates this expectation and
we are not surprised when he tells Mary that he is going to leave her.

There
is in what Patrick does since Dahl has painted his wife as an
innocent and harmless individual who needs protection. This aspect is emphasized by the fact
that she is pregnant with his child. She is in an extremely vulnerable position and Patrick
should, therefore, be more supportive of her. This is not only true because he is a husband,
soon-to-be-father, but also because he is a detective. This means that he is there for the
protection of the weak and should be selfless. Patrick is, however, uncaring and thinks only of
himself.

The irony is extended throughout the
story. Mary commits a most heinous criminal act. She clobbers her husband to death and then
deliberately proceeds to cover up her crime. Her actions speak of one who is cold-hearted and
vengeful, not one who would exude a look of peace and calm as described earlier:

...she was curiously peaceful. Her mouth and her eyes, with their
new calm look, seemed larger and darker than before. 

Mary convincingly plays a charade and fools everyone into believing her. The visiting
detectives and other officers literally eat out of her hand, so much so that she lets them
ingest the murder weapon. In this instance, Dahl uses both verbal and situational
irony as well as sardonic humor to make the point
that appearance and reality are not always one and the same thing; we may be easily
deceived.