Stark. The Dark Half.

For years, Thad Beaumont has been writing books under the pseudonym George Stark. When a journalist threatens to expose Beaumont's pen name, the author decides to go public first, killing off his pseudonym. Stark isn't content to be dispatched that easily, though. Beaumont's alter ego comes to life and begins to stalk those responsible for his demise. The police suspect Beaumont is responsible for these violent crimes. (source)