Being a universal peacekeeping force, the Green Lantern Corps is structured very much like a formal military. While they claim the role of peacekeepers, they employ a number of methods to protect the universe, some of them more dubious than others. In 2006, Green Lantern Corps #7 (by Keith Champagne, Patrick Gleason, Prentis Rollins, Moose Baumann and Phil Balsman) unveiled the secret "dark side" of the Green Lantern Corps, and it's questionable to say the least.
Of course, every formal military has a black ops division: a select group of soldiers who leave their morals at the door to complete the missions that most others are too ethically encumbered, shall we say, to complete. For the Green Lantern Corps, this is the the Green Lantern Corpse, and while they are powered in a way similar to their shiny brethren, the energy disks they ingest to wield the power of will are very different... and a lot darker.
The principle behind the the Corpse is simple, a select group of Lanterns are placed within a secret division no other regular Lantern knows about for missions too dark for a typical lawman to complete. To ensure their anonymity, the memories of the regular Green Lanterns are altered to forget those inducted into the Corpse or to make them believe that they are dead. These Corpsemen don't wield the standard ring, their missions are too delicate and require absolute stealth; they also need to deny responsibility. As such, they don't wear their rings, they eat them.
Truthfully, these aren't even rings at all, but rather, small energy disks that can channel the power of will for approximately five Earth days. It's safe to consume, provides a decent amount of charge and it's entirely discreet, without the usual limitations an ordinary ring possesses or its distinct hue; instead, it's more of a violet color. The energy disks also have no noticeable insignia connecting them to the Green Lantern Corps, and the suits they produce are completely black, which of course is ideal for stealth missions like, say, assassinations.
This makes the disks sound better than the rings, but there are some limits. The Corpse disks have a limited charge and they only get one per mission. If they fail their objective before the charge runs out, they are typically left to die; that's just part of the deal -- more power, less accountability, but a lot less backup.
Normal Green Lanterns act as space police in the DC Universe, but they can't go around murdering their enemies indiscriminately. The Corpse has no such limitations, and in fact, most of their missions revolve around infiltration, espionage and assassination. That's why the disks are not limited by the rings' usual mandate of not using lethal force, which admittedly was later changed after the results of the Sinestro Corps War.
It's a rather intimidating notion that the Guardians of the Universe have (or had) a personal group of assassins to deploy at any given moment. Fortunately, it seems that to keep this group secret, only a handful exist at any given time. Last time the Corpse was seen, only two were active, though this was when the Corps was rebuilding itself, so it is entirely possible that more Corpsemen may exist right now. Even though they haven't been seen since 2006, given that they can erase themselves from the memories of others, it is also entirely possible that the division still operates within the present day Infinite Frontier. If so, they may be out there, readying themselves to engage in the missions where the Green Lanterns' light cannot shine, and that could be dangerous.