Because Voldermort expects the prophecy to become a reality if all the conditions it states are met, e.g. hermione dying. It doesn't matter how he tries to thwart Harry, as long as Hermione dies the prophecy will be fulfilled and Harry wil end the world. He believes this strongly because he's tried it before himself and there is plenty of lore of it happening before.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 112.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: