ANSWER:
This question was adopted from a short story by Roald Dehl called "LAMB TO THE SLAUGHTER".
The carful plan Mary did in the story, was to cover up her murder crime, by plotting a setting that will give her the ability to tell a deceitful story, that won't make her a suspect about the death of her husband; Patrick.
But her careful plans influenced the outcome of the story, by defining Mary to be a deceitful wife, which knows how to fake a life, instead of the definition of a loving wife which she has at the beginning of the story.
Mary has shown herself to be very good in faking a life story, and this was one of the reasons why her husband was not comfortable with the marriage, and wish to divorce. All the love Mary showed to Patrick in the beginning of the story was not real, it was all a fake. Mary only wanted Patrick because she doesn't want her baby to grow without a father, that was why even when she killed Patrick, her baby came first in her considerations, as she doesn't want to take chances of making her baby an orphan. This is why she wants to protect herself from the crime, in order not to leave her baby growing without father nor mother.
Form the whole story. It can be assumed that the couple's doesn't love each other anymore, but Mary was trying to keep the relationship active, because of the interest of her unborn baby. This is the reason why it was very easy for her to kill Patrick with courage, and tries to cover up.