Why do you think Vera told Frampton the lie that she does in "The Open Window"?
In Saki's "The Open Window" the author writes with his usual sparkling wit and irony; he offers a frame story for Vera's humorous manipulation of Framton Nuttel. Saki's mischievous narrator is perspicacious enough to identify people's vulnerabilities while seizing on details around her and quickly weaving a convincing tale that can deceive her listener and exploit his weakness with her subtle ridicule and practical jokes. Perhaps, too, because she is sent to entertain guests by her aunt far too often, Vera conjures such tales in order to entertain herself.
When Vera notices that Framton is nervous and ambivalent as he
...endeavored to say the correct something that should duly flatter the niece of the moment without unduly discounting the aunt that was to come...
she decides to amuse herself by fabricating a tale that will end at Nuttel's expense. The fabrication that she uses is initiated by Framton himself because he asks about the open window: "...but has that window got anything to do with the tragedy?" Knowing that the men will return through this window just as they have departed by passing out its opening, Vera decides to weave her tall tale around the hunting expedition. And, because "[R]omance at short notice" is her forte, Vera succeeds at her practical joke; after seeing the "dead" return through the window, Framton races away, to Vera's satisfaction.
Vera saw that Frampton was much weaker than she was. She saw him as being a good listener, and since she loved telling tall tales, she also saw his potential to become a good listener and a victim of her little lies and pranks. She had not really wanted to deceive Frampton, she was just a creative, young, and dramatic teenager.