Any Green lantern is not permitted, or was not permitted, to kill any sentient being. The Rings are programmed to shut down if it senses an attempt to purposely kill by it's host. Recently however, during the Sinestro Corps. War and afterwards, the Guardians of the Universe reveal two additional laws to the Book of Oa that allow Lanterns to kill members of the Sinestro Corps., and then any enemy to the Green Lantern Corps. These laws are controversial however, and have led to at least one court martial for inappropriate killing of a captured Sinestro Corpsman.