Is it appropriate for the wife to kill her hubby in "Lamb to the Slaughter?"

The author does not give us much information about why Patrick is leaving his wife: 

And I know it's a tough time to be telling you this, but there simply wasn't any other way. Of course, I'll give you money and see that you're taken care of. But there really shouldn't be any problem. I hope not, in any case. It wouldn't be very good for my job. 

We know that it takes four to five minutes for him to tell her the reasons. And we know that Mary sits, listening to him "with puzzled horror." The earlier paragraphs indicate that Mary is pregnant, happy, and dedicated to her husband. Naturally, this news comes as a shock to her. 

When he informs her of his decision, she is justifiably upset. If he is leaving her because he is bored with the marriage, she is right to be upset. If he is leaving her for another woman, she is right to be even more upset. And with a child on the way, his timing could not be worse. However, none of these scenarios really justifies murder. She would be justified in publicly and privately shaming him, taking half of his money, and maybe even suing him. But murder is going too far. 

But note that we really do not know what Patrick says to his wife. For the sake of argument, suppose something absurd. Perhaps Patrick has lost his mind. He then informs his wife that he is leaving her to go on a killing spree. If something like this was said, she would be completely justified in killing him. But this is a stretch. 

This story is like a dark fairy tale. Although in most scenarios, murder is not justified, it is morbidly satisfying when Mary kills her husband. While most readers would not justify it, ethically and legally, in real life, there is a warped sense of justice when she kills him and has the detectives eat the evidence. 

Is it appropriate? Most likely, it is not. It all depends upon what Patrick said. And we have little to go on with that.