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. She might just as well have hit him with a steel club.
These words in Roald Dahl's short story "Lamb to the Slaughter" make it crystal clear that Mrs. Maloney is legally guilty of murder.
Whether she is morally guilty is another matter. It is clear that her husband is going to leave her, but we never hear anything of the details. Who is to blame is entirely a matter of conjecture. He promises to give her money and ensure that Mary is looked after, but his dispassionate way of talking may strike many readers as terribly cold-hearted, like the reflection that it wouldn't be very good for his job.
However, even though the story is focalized through Mary's perspective, nothing her husband says or does could be taken as justification for murder—certainly not legally and almost certainly not morally either. The last line of the story suggests that Mary feels no remorse and is rather amused by her own cleverness in getting away with her crime.