How precisely do we relate this statement to this excellent short story? Firstly, I suppose the revelation of Patrick to his pregnant and doting wife Mary could indicate human cruelty, but I think we need to take a more nuanced view when we come to consider Mary's response in killing Patrick. The text makes it completely clear that such an action was involuntary and was something that Mary had no control over herself:
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.
The automatic nature of this action and the way that she comes out of shock after having done it indicates that this was an unthinking response to the terrible situation she was facing. Clearly, this could be used to argue that deep down in Mary's being, the same violence and cruelty exists that other humans have, and the shock of finding out that Patrick was leaving her unleashed these emotions. More importantly, in my opinion, is the way in which Mary Maloney very calculatingly organises the disposal of the murder weapon and the removal of the suspicions against her. The reference at the end of the story to Mary Maloney "giggling" as the detectives eat the murder weapon whilst talking about where it is definitely seems to suggest that there is a perverse, cruel and violent nature to all of us.