What is Saki's tone and style in general and in "Sredni Vashtar" in particular?

In general, Saki employs irony in his stories along with satire of the foibles of Edwardian society. In his short story "Sredni Vashtar," the author adopts an ironic tone and he satirizes in an imaginative and macabre style the oppressiveness of Edwardian society as represented by the character Mrs. De Ropp.
"Sredni Vashtar" is a story of a boy's efforts to survive. While the doctor has pronounced that he will die within five years, Conradin's greatest adversary is Mrs. De Ropp, his rather disagreeable cousin and caretaker. Because he is aware of her antipathy,

from the realm of his imagination she was locked out—an unclean thing, which should find no entrance.

Conradin's goal each day is to enjoy things Mrs. De Ropp does not approve of, and to find a place where he can privately go. He chooses an abandoned tool shed where a Houdan hen lives. Conradin treats the hen affectionately because he has little else to love. In a hutch, Conradin also has a large polecat-ferret, smuggled in by a butcher-boy. 

Its very presence in the tool-shed was a secret and fearful joy, to be kept scrupulously from the knowledge of the Woman, as he privately dubbed his cousin. 

Because he has little else, Conradin, who actually fears this animal, assigns it a special significance. Its presence becomes a "fearful joy" because the Woman has no knowledge of its existence. In time, the boy's imagination makes the creature into a god with the name Sredni Vashtar, and Conradin holds festivals of worship for it whenever possible. When Mrs. De Ropp takes Conradin's beloved hen from him, Conradin prays to his little god, "Do one thing for me." 
After a while, the Woman notices Conradin has not stopped his visits to the shed even though the hen has been removed. She asks him,

"What are you keeping in that locked hutch?" she asked. "I believe it's guinea-pigs. I'll have them all cleared away."

Conradin continues his prayers as Mrs. De Ropp goes out to the shed. Time passes and the maid brings in the tea, wondering where her mistress could be. After a while, Conradin hears the maid scream. Conradin then hears,

"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.

Once again, Saki ironically disposes of the adult. In this story, the adult meets a horrific death, a death depicted as a something in which the boy delights as he calmly continues to butter his toast.