Note that the translation of the opening of the prayer in the King James version and the English Book of Common Prayer is:
> Lord, now lettest thy servant depart in peace
and "depart" refers originally to the death of Simeon. It is traditionally used as a night prayer. In literature it is often quoted as part of a reference to death, or to a character exiting a play or novel. It sometimes refers to the retirement of a person after a long career. Dahl would have herd the prayer often as he grew up, and encountered many literary references to the words "Nunc Dimittis".
Note that at the end of the story the narrator (Lionel) is feeling ill, and there is a clear implication that he has been poisoned by his victim, Janet. So one may say of him "now let him depart" for he is getting the release that he has earned, poetic justice.