In the Green Lantern film, Sinestro asks that the Guardians forge a yellow ring:
Sinestro: With my own eyes, I saw this creature produce fear in our finest warriors and then feed on that fear! There's more: we tracked its trajectory. Parallax is headed to Oa! Never before has an enemy dared to attack us here! You MUST tell me what you know!
First Guardian: Will has always been our sole weapon against the forces of darkness in the universe. But as that power began to seem insufficient, a great debate arose among us. Should we exploit another source of power, one we had long since sworn should never be used... fear? But the power of fear was too unpredictable, the chance of corruption too great. And so we decided against it. All of us, that is, except one. Alone, this Guardian entered the forbidden chamber. He wanted to prove the light of fear could be mastered. His intentions were pure, but his hopes were naive. Consumed by the yellow power of fear, the Guardian became the very evil he had wished to destroy. He became Parallax. For the universe's safety and his own, Abin Sur was charged with imprisoning Parallax in the Lost Sector.
Sinestro: But he escaped.
First Guardian: Feeding on the fear of others, he grew larger, more powerful...
Sinestro: ...and he's now headed here to take his revenge on you, and destroy the Corps!... Then we have no choice. We must prepare to harness the power of our enemy, and fight fear with fear. To protect our sectors, we must be able to defend ourselves. We must forge a yellow ring.
Then later, after the ring is forged, he says:
Sinestro: Our new weapon is ready. Once I've mastered it's power, I'll begin training the Corps to do the same. Their worth will be lost. We'll be ready to make a stand against Parallax before it reaches Oa. And the power of the enemy will be ours.
There seem to be a couple of flaws with Sinestro's thinking
- A Guardian was unable to master the yellow power of fear, and ended up becoming the personification of fear. What makes Sinestro think he'd do any better?
- How would fighting fear with fear accomplish anything? Parallax feeds off fear, so even if the goal was to produce fear in Parallax, wouldn't that have just made him stronger, because he could feed of his own fear?
(This seems to be roughly the same problem as the people of Middle Earth thinking that they could use the One Ring against Sauron)