Should is a very different verb from will. The end of this excellent short story makes it very clear that Mary Maloney has managed to get away with the murder, as she involves the policemen in hiding the murder weapon for her permanently as they eat the leg of lamb that she has just roasted and given them to eat.
However, if you are looking at the evidence from the reader's point of view, I think there is very definitely a case for her getting off with temporary insanity or something like that. Note how the actual murder is described:
At that point Mary Maloney simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head.
It is clear that this was unpremeditated, and was brought around by the shock of discovering that he was going to leave her. We are also told that she was in "shock" with what she did. Clearly, her position as a pregnant wife who has just found out that her husband is probably having an affair and leaving her for another woman would have enabled her to get off in the hands of a good lawyer. But remember - the beauty of this darkly humorous story is that she is able to provide the perfect alibi for herself and destroy the murder weapon.