Why did Arthur Conan Doyle kill Sherlock Holmes?

Sorry for the spoiler, but I assume it’s common knowledge at this point. I recently read all of the Sherlock Holmes stories and I discovered that there is a whole group of people who really care and know about Sherlock Holmes, I’m hoping there will be a couple on Fluther. Reading the introductions and notes on various volumes of Holmes stories, it seems that everyone takes Doyle at his word that he killed off Holmes because he was tired of writing about him and never intended to bring him back. But one characteristic of the stories is that every detail, particularly every utterance or odd behavior of Holmes, will eventually play a part in the story and be explained. Nothing is left hanging. But when Holmes dies he has told Watson he is afraid of air guns. This is never addressed again in the story. Then he conveniently dies with no witnesses and no body. Then in the return of Sherlock Holmes an air gun plays a key role. It seems to me that Doyle planned from the beginning to bring Holmes back. Why then do the scholars and experts, who must know more than me, take Doyle at his word?