What makes the ending of "Sredni Vashtar" powerful?

The ending of “Sredni Vashtar” is powerful in many ways. First, it is a very cathartic ending, where the reader can finally feel some vicarious relief through the eyes of the main character, 10 year-old Conradin.

Conradin is a child who leads a terrible existence. Terminally ill, unloved, and badly treated by his guardian, Conradin’s daily emotions shift, from hatred toward Mrs. De Ropp, to an intense fascination with the things that he conjures up in his imagination.

One of his most powerful ideas is about a ferret that he names Sredni Vashtar and that lives in a "disused toolshed" in a "dismal" forgotten corner of the "cheerless" garden overlooked by the "many windows" of his house. One day, out of "Heavens knows what material," the ferret becomes Conradin's god to whom he offers prayers and festival gifts of nutmeg.

After Mrs. De Ropp has the Houdan hen sold as a result of her noticing how much time Conradin spent in the toolshed, his prayers to the ferret-god change from giving praise to asking a "boon," a life-improving kindness, though not explained in detail: 

   Conradin had been wont to chant his praises, tonight be asked a boon.
   "Do one thing for me, Sredni Vashtar."

The ending is powerful because it provides the cathartic, yet, tragic moment the main character has secretly prayed for. Another reason it is powerful is because the tragic end is presented with an ironic twist: the prayer fulfillment happens during a typical, daily moment in Conradin’s life, tea time.

Making himself toast, which was always forbidden by his guardian, Conradin enjoys this first taste of freedom while at the same time he:

….. listened to the noises and silences which fell in quick spasms beyond the dining-room door.

Even though both characters have animosity against one another, Conradin’s guardian is comparatively worse because she actually gets to be mean and careless toward him. Therefore, the narrative is partial to Conradin, and the reader waits for some sort of restitution at some point. This restitution is the key reason why the ending is so powerful.

Saki only suggests Conradin’s emotion at this point. It is clear from his actions, which Saki describes, that the boy's emotions are not what would be expected of a normal boy in a normal situation because he quietly and deliberately goes to tea.

Conradin fished a toasting-fork out of the sideboard drawer and proceeded to toast himself a piece of bread. And during the toasting of it and the buttering of it with much butter and the slow enjoyment of eating it, Conradin listened ... the loud foolish screaming of the maid,....

Conradin helping himself to another piece of toast while her body is being loudly mourned and carried into the house adds to the irony of the situation. Not only has finally avenged himself through karmic justice, but he finally gets the pleasure of enjoying an extra helping of toast, while those in the household believe that this will be a tragedy to the child.  

   "Whoever will break it to the poor child? I couldn't for the life of me!" exclaimed a shrill voice. And while they debated the matter among themselves, Conradin made himself another piece of toast.

The ending is powerful because it is cathartic, enigmatic and ironic.

The ending of Shredni Vashtar is so powerful because it illustrates the horrific glee and the quiet satisfaction of a child at his guardian's death. The resolution is made even more poignant when we realize that Conradin views his guardian's death (at the hands of his pet ferret) as the means to his liberation and self-autonomy. 

When Mrs. de Ropp retrieves the key to the hutch, Conradin prays to his sole champion and protector, the polecat-ferret, for relief from the machinations of his meddling guardian. However, the author tells us that, even as he prays, Conradin fears that his prayers will go unanswered. This is because his guardian has always prevailed over his childish will in past skirmishes.

However, as time continues and his guardian does not emerge from the shed, Conradin is hopeful. We do not know what he is hopeful for until the author describes the emergence of Shredni Vashtar with 'dark wet stains around the fur of jaws and throat' from the shed. Presumably, the wet stains represent the bloody aftermath of a skirmish between a human and an animal, with the animal emerging as the clear victor. The text states that 'Conradin dropped on his knees' at this sight. This clear indication of relief and happiness at answered prayer shocks us. Did Conradin pray for his guardian's death?

The author doesn't confirm this, but the story ends with Conradin calmly making himself another piece of toast. This is a powerful ending illustrating that anyone (even a child), when pushed to his limit, may respond in an uncharacteristic and malicious fashion.

What makes the ending of "Sredni Vashtar" by Saki so powerful?

The ending of Saki's "Sredni Vashtar" is powerful because it has elements of the mysterious and the preternatural that converge with the realism of the story.

Saki's narrative is rife with intense feelings, though many of which are masked. Mrs. de Ropp is the "uninspired" guardian of ten-year-old Conradin,  a sickly, but imaginative boy who "hates this cousin with a desperate sincerity" and is in "perpetual antagonism" with her. She feels it is her duty to thwart the boy "for his good," and it is a duty that she does not find "unpleasurable." With such tension between this stringent adult and imaginative boy, there is generated a tension which becomes almost palpable.

In order to subvert his cousin and to have something on which to pour his affection Conradin keeps a little Houdan hen in a half-hidden, unused tool shed to which his guardian gives no attention. Farther back in the gloom of this old shed is later hidden a "polecat-ferret," surreptitiously brought in for Conradin by a butcher-boy. To this ferret Conradin gives the exotic name, Sredni Vashtar

...for he was a god who laid some special stress on the fierce impatient side of things, as opposed to the Woman's religion, which, as far as Conradin could observe, went to great lengths in the contrary direction.

Every Thursday Conradin holds a "mystic and elaborate" ceremony before the hutch of Sredni Vashtar, "the great ferret." He celebrates some ailment of the woman such as a severe toothache, and offers the ferret some stolen nutmeg in homage. 

After some time and to Conradin's dismay, his guardian discovers his visits to the Houdan hen, and she has the unfortunate bird taken away. In protest, Conradin sets his face stoically and declines without emotion to eat his toast at tea time. 

"I thought you liked toast," she exclaimed, with an injured air, observing that he did not touch it. "Sometimes," said Conradin.

Later, when he has the opportunity, Conradin visits the tool-shed:

...in the evening there was an innovation in the worship of the hutch-god. Conradin had been wont to chant his praises; tonight he asked a boon.

In supplication to Sredni Vashtar, Conradin requests that the "polecat" avenge the loss of the hen. But, he only says, "Do one thing for me, Sredni Vashtar." The boy leaves it up to his revered god to act according to its nature. With a sob for his lost pet, Conradin returns to the house. Every night Conradin repeats to his god, "Do one thing for me."

Finally, Mrs. de Ropp notices that Conradin still goes to the old shed. Angrily, she asks the boy what he yet has in there. After she discovers the hutch, she tells Conradin that she suspects that he has guinea pigs; these, she declares, will be cleared out the next day. In despair, Conradin feels certain that the woman will go in the shed the next day and come out in triumph. His ferret, a god no longer, will then be carried out by the gardener. In despair Conradin chants loudly and fervently "the hymn of his threatened idol":

Sredni Vashtar went forth, His thoughts were red thoughts and his teeth were white. His enemies called for peace, but he brought them death. Sredni Vashtar the Beautiful.

The boy's intense belief in his god, his powerful feelings of hatred, and the innate nature of the polecat converge as does happen in life. Of course, realistically, the intrinsic nature of the ferret probably causes the demise of Mrs. de Ropp, but there is a possibility of the power of the human will as a factor. What also takes the ending further than reality is the cool and preternatural indifference displayed by Conradin when he discovers that Sredni Vashtar has, indeed, avenged him.