Before he embraced the power of fear and became the Green Lantern Corps’ greatest enemy, Thaal Sinestro was a Green Lantern himself.
Unfortunately, not only did Sinestro manage to return, but he’d obtained a yellow power ring from the Weaponers of Qward that functions quite similar to a Green Lantern ring, but feeds off the wearer’s ability to instill fear rather than their willpower.
When the Guardians of the Universe learned that Sinestro was using his ring’s power to instill fear in Korugar’s citizens, they expelled him from the Corps and banished him to the antimatter universe.
Sinestro was determined to instill law and order so much on his homeworld of Korugar that he ruled the planet as a dictator.