In the Green Lantern movie, there was an after-credit scene which displayed that Sinestro put on the yellow ring of fear.
Sinestro wasn't a dumb guy. He was one of the smartest Lanterns in the universe. And, he knew that even a Guardian was unable to control power of fear. Power of fear was unpredictable unlike power of Will.
Initially, he agreed to master power of fear because there wasn't another way in front of him to beat Parallax. But, why did he try it at the end, when Parallax had already been defeated by Hal Jordan (who showed that Will was more powerful than Fear)?