Mary is a completely devoted wife. She adores Patrick and waits on him like a maid. She is doting, attentive, and effusive. This may be why Patrick decides to leave. We are never given a reason. He may have been having an affair as well. The author never makes this clear. As for Mary, she feels completely comfortable and satisfied with her life with Patrick. She is six months pregnant and it would appear that she has complete faith that their future family will be a happy one as well. She is so happy, dependent upon, and we might even say addicted to Patrick that when he informs her he is leaving, she cannot handle it.
The author doesn't give a word-for-word dialogue of how Patrick tells Mary he is leaving her. But by the end of it, she knows the marriage is over and there is no hope of reconciliation. She is stunned and then reacts quickly. In her shock (or conscious reaction), she kills him. Immediately after this, she becomes a clear-thinking detective, determined not to be caught. She carries out her plan to perfection. When the detectives eat the evidence, she laughs in the other room. The seemingly perfect, devoted wife reveals a sinister side in this last line. She is a much more complicated and mysterious character than the opening paragraphs would suggest.