The Guardians of the Universe chose their Green Lantern Corps members based on several requirements: those worthy must have strong wills and must possess the ability to overcome great fear. While the majority of Green Lantern members are heroes like Hal Jordan, Kyle Rayner and John Stewart, the power rings have chosen several villains like Sinestro and Harley Quinn.
Source：https://www.cbr.com/dc-comics-best-villains-green-latern-rings-worthy/
