The proof that Vera's trick on poor Framton Nuttel is unjustified is shown by the fact that she invents another story to protect herself from being punished. No doubt the adults would eventually suspect that Framton ran off because of something Vera had told him about them. This cannot be the first time that this mischievous girl has played a cruel practical joke. If they couldn't get the truth out of Vera, they might even make inquiries. Since Framton had brought a letter of introduction from his sister, it would be natural to write her a letter in reply. But Vera tells them a fantastic story about Framton having developed a phobia of dogs.
"I expect it was the spaniel," said the niece calmly; "he told me he had a horror of dogs. He was once hunted into a cemetery somewhere on the banks of the Ganges by a pack of pariah dogs, and had to spend the night in a newly dug grave with the creatures snarling and grinning and foaming just above him. Enough to make anyone lose their nerve."
We can imagine how Mrs. Sappleton and her husband would have reacted if they had known the real truth about the incident. They would have been horrified, especially since Framton was visiting under the auspices, so to speak, of the local vicar. They would have written letters of apology to the vicar, to Framton's sister, and one to be forwarded to Framton himself. Vera would have been punished. The author twice describes her as a very self-possessed young lady. She knows she could get in serious trouble for a serious offense, but her self-possession and her truly fertile imagination save her.
"The Open Window" is still a funny story in spite of everything.
Vera bears some resemblance to thirteen-year-old Briony Tallis in Atonement by Ian McEwan. McEwan's excellent novel is extensively covered in eNotes. See reference link below.
This is an excellent question for evaluating the character of Vera in Saki's short story "The Open Window."
From the very beginning of the story, we know enough about Vera to form expectations regarding her behavior. She is
...a very self-possessed young lady of fifteen.
Self-possessed is defined as
...having or showing control of one's feelings, behavior, etc.; composed; poised.
The age and gender of our protagonist is essential to the story's success.
…girls generally appear to be better behaved…[her age is important because] she had to be old enough to be entirely credible.
During the early twentieth century when Saki (Hector Hugh Munro) was writing, social expectations for a young woman of good character were well-defined. At fifteen, Vera would have been old enough to welcome a visitor and entertain him until her aunt arrived. She would know all of the socially accepted behaviors that would meet the Victorian era's staunchest critics. She would have been expected to be polite to the extreme. In keeping with the societal norms of the time, one could infer that by her sixteenth birthday, she would be introduced into society as a young woman in search of making a good marriage—as was expected of her. In other words, she is old enough to know better.
For these reasons, I find that "teasing" is much too light a word for behavior that is malicious and intentionally cruel.
Upon meeting Framton Nuttel, Vera would easily have been able to assess his lack of self-confidence. As mentioned earlier, society would have prepared Vera to interact with ease in the company of all kinds of people; even living in the country (out of the crush of social gatherings one might expect if residing in the city), she would have understood what was expected of her.
Malicious intent is defined as...
...the intent [purpose], without just cause or reason, to commit a wrongful act that will result in harm to another.
Vera's intent here is not to simply tease their guest (which would have been impertinent behavior toward a stranger in her aunt's house), but to scare him to death simply for her own malicious amusement.
The author details Vera's preparations to carry out her scheme:
"Do you know many of the people round here?" asked the niece, when she judged that they had had sufficient silent communion.
"Hardly a soul," said Framton. […]
"Then you know practically nothing about my aunt?" pursued the self-possessed young lady.
"Only her name and address," admitted the caller.
This line of questioning seems innocent enough to one who is unaware of the story's ending. Vera's introduction into their conversation of "her [aunt's] great tragedy" is the pivotal point of the story. It is at this moment that Vera intentionally lays the groundwork to frighten Framton. There can be no doubt that Vera is well aware of the circumstances surrounding her uncle's habit of hunting and returning at a particular time of the day. With this knowledge and with an understanding of the fragile nature of the nervous man before her, Vera not only expertly sets the stage for the men of the family to return as if ghosts across the yard, but she also is able to feign her own horrific response to the sight of the presumably dead men approaching them through the open window.
In essence, I see no justification for Vera's behavior. Framton would bear the scars of such an experience long after returning home from the country—which was, ironically, supposed to settle his nerves. Vera would have enjoyed her afternoon's amusement and moved on without any sense of contrition. In fact, I find it all the worse in that she would ostensibly never be found out for her behavior and would never be chastised or punished.