What is the tone of "The Appointment in Samarra" by W. Somerset Maugham?

The famed English poet W. Somerset Maugham wrote this interesting version of this, said to be an ancient Mesopotamian fable or folktale. The tone is fascinating because it is deceptively simple and somewhat misleading because of the point of view. The author clearly indicates the point of view with the centered line that begins the fable: "The speaker is Death."
Death relates the story of the Baghdad merchant who sends his servant to the marketplace, and the servant recounts his frightening experience there, seeing Death. Death is a woman, and the servant says that Death jostled him and "made a threatening gesture." The servant asks the merchant to lend him a horse so he can ride to Samarra, to escape Death's plans for him. Death then says the merchant went to the marketplace and "saw me in the crowd" and asked about the incident:

Then the merchant went down to the marketplace and he saw me standing in the crowd and he came to me and said, Why did you make a threatening gesture to my servant when you saw him this morning?

This sentence offers perhaps the most important clue as to the tone of this short piece of writing. Death, the speaker, has an omniscient viewpoint and begins in what seems to be the third person but here abruptly switches to a first person narrative. In the final line, the story's surprise and irony are revealed—that Death was not threatening the servant, but merely reacting with surprise to see him in Baghdad, because she had an appointment with the servant "tonight in Samarra."
But even though the switch to a first person perspective and the revelation of the appointment reveal Death's all-powerful perspective and control over human fate, there is a slightly confusing conundrum here. If Death is in control of the servant's fate, why is she not able to predict the servant's actions as well? This suggests Death is not quite all-powerful after all, yet it also suggests that the fate of death, referred to as an "appointment," is inescapable and final.
The story is therefore slightly confusing and mysterious, despite its simplicity and brevity. Its tone feels very straightforward, but the information revealed is ultimately very somber and ominous.