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.)
Sinestro, at the time, was the leader of the Green Lantern Corps and regarded to be one of the greatest Green Lanterns ever. He also had a very big ego and believed that he could accomplish what Parallax had failed to do: control the yellow essence of fear without losing himself to it. He also believed that the green essence of will power was no match for the yellow, and that if he wanted to defeat Parallax he would have to use the same power, which is why he asked for a yellow ring to be constructed.
Had Sinistro's plan prevailed he would've worn the yellow ring into battle with Parallax. The latter wouldn't be able to siphon fear from Sinstro's ring because the yellow ring didn't emit fear, it absorbed it, the same way Parallax does.