In the book (IIRC): Smith sees that Stillson is responsible for the end of the world (basically) and decides to assassinate him. He hides overnight at the rally site with his rifle and tries to kill him during his speech. He fails, though. In desperation, Stillson manages to avoid getting...
I'm glad they worked this episode this way. It gave me closure in regards to how the book ends. Now I can enjoy where ever the series leads knowing his destiny won't follow suit..