Based on the passage, why does the author include the scene where Mary talks to herself in the mirror?

When Mary speaks to herself in the mirror, the reader is able to get a glimpse of Mary when she is perhaps her most authentic self. She realizes immediately after killing her husband that the punishment for murder will be severe, and she thinks frighteningly quickly about how to save herself. This scene in front of the mirror reveals Mary's cold clear-mindedness about what she has just done, which rules out the possibility that Mary's murdering her husband is a simple crime of passion.
Before this mirror scene, the reader might think of Mary as someone who is impulsive, someone who lost temporary control of her emotions and made a huge mistake, misjudging the damage a frozen leg of lamb could do. After the mirror scene, the reader must question Mary's sanity; she seems completely capable of murder as she tries out different voices to make herself seem normal as she goes out to seal her alibi. The care she takes to make sure she sounds like herself also inspires curiosity in the reader: could the murder have been premeditated, on some level? The mirror scene is indeed an important one, as it helps to flesh out the character of Mary.