In "The Open Window," why does Vera have to deceive Mr. Nuttel?
Saki’s short story “The Open Window” features a literary technique called dramatic irony. A writer creates dramatic irony when he/she reveals information to the reader that one or more of the characters in the story does not know. It's a good way to create suspense and make a reader think about how characters will react to the circumstances of the story.
When Vera tells Mr. Nuttel about the missing men, the reader eventually discovers that this story is a lie. Because Mr. Nuttel does not know it is a lie, we have an instance of dramatic irony.
Why does Vera have to lie? Well, for Saki to create the intended effect (dramatic irony) the reader needs to learn something that the character does not, and the lie makes this possible.
Notice that Vera does not only tell one lie. After Mr. Nuttel runs away, she lies about the reason he left, saying that he was afraid of their dog:
"I expect it was the spaniel," said the niece calmly; "he told me he had a horror of dogs.”
This lie shifts the focus from Mr. Nuttel’s situation to Vera's behavior. Saki’s theme now becomes clearer. Instead of a central message about a man who is trying to deal with a nervous disorder, the reader is presented with a girl who deceives impulsively, and apparently just for the fun of it. The second lie forces us to confront the question: what does Vera’s deceitful behavior say about humanity?
In the story The Open Window, Vera (a fifteen year old girl) is waiting in her home for some relatives when she receives he visit of Mr. Frampton Nuttel, who was visiting the country side as a way to cure his nerves, and was going to spend time with them.
Vera tells him a story about tragedy and romance, drama, and suspense about her relatives that totally enthralls Mr. Nuttel. Yet, to answer your question, Vera did not have to deceive Mr. Nuttel: She simply did it because, as Saki says at the end of the story, Vera's specialty was to tell stories which were "romances at short notice."
In other words, Vera simply could not resist. She saw that the man was, in essence weaker than herself. She saw in him a potential good listener to a made up story, and since she loved telling such stories, she also saw in him a victim of her little pranks.
Hence, to answer your question Vera didn't much HAD as much as she WANTED to deceive Mr. Nuttel as a young, picaresque and creative, dramatic teenager that she is.
There is unclear reason why Vera has to tell him a lie. But, possibly, she does it because it's her talent as the writer indicates at the end of the story and her characteristics. Additionally,she finds that Mr. Nuttel is a outlander and easy to hoax. it seems that she often does it as she lies again when her relatives ask why Mr. Nuttel has to run away when they come up. It might be she is just fifteen years old, the age that'd rather live in the imaginative world