|That's not a good attitude...|
Due to events in "Reign of the Supermen", Coast City, the home of the Hal Jordan Green Lantern, was destroyed. Hal attempted to use his powers to recreate Coast City, but this upset the Guardians of the Universe (his bosses) since he was doing this for personal gain. As such, the Guardians ordered Hal to give up his ring. He flew off to the planet Oa (the base of the Green Lanterns and Guardians). On his way there, several Lanterns attempted to stop Hal. However, Hal killed them and stole their rings to make himself more powerful. Desperate to stop Hal, the Guardians gave Sinestro, a former rogue Green Lantern, his ring to stop Hal. Hal killed Sinestro and Hal's old pal Kilowog. Hal absorbed all of the power in the Central Power Battery (the source of the Green Lantern's powers) and destroyed it. Hal became the villain Parallax. All the Guardians, except Ganthet, are killed by the explosion caused by the battery's destruction. Ganthet went to Earth and gave the last Green Lantern ring and battery to Kyle Rayner (who became the then-new Green Lantern).
The original script of "Emerald Twilight", written by Gerard Jones, was vastly different than the final product. Originally, a new group of altruistic aliens appear and declare themselves the Guardians of the Universe and tried to take power away from the pre-existing Guardians. The Green Lanterns would have choose which were the true Guardians of the Universe. DC Editorial supposedly thought it wasn't interesting enough. As such, various editors came up with a basic outline for a new story and Ron Marz wrote the draft that got published.