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.