Given the task definition and input, reply with output. Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

In the movie Green Lantern, if fear is the very thing that turned the original Guardian into Parallax, and it cannot be conquered (The Guardians themselves say so)... why then does Sinestro decide to have The Guardians create a ring of fear? If the Lanterns fought Parallax with fear... would he not just feed off of the fear from the ring? Not to mention, wouldn't the fear take control of any Lantern that tries to master it within the yellow ring? (Example: Sinestro in the end credits.)
Why does Sinestro have The Guardians create a ring of fear?