Green Lantern #7 (May 1961) debuts another long-standing Green Lantern villain: Sinestro. The Guardians explain to Hal Jordan that Sinestro was the Green Lantern of sector 1417 on the planet Korugar. I guess they should have thought twice to give a power-ring and battery to a being with such an obviously-evil name, but oh well.
It turns out Sinestro was a really strong Green Lantern but quickly got bored with his setup and tried to become the planet’s dictator instead, an example of absolute power corrupting absolutely. When the Guardians discovered this, they summoned him, stripped him of his ring and battery, and banished him to the antimatter universe of Qward.
Of course Green Lantern outwits the Weaponers of Qward and Sinestro, imprisoning him in a green bubble (for eternity?).