Oh, agents.
Cute.
And no real offense taken. And personally, I disagree with the "not purely selfish" thing, since, well, I created this theory;
It seems I need to drag out my own theory on the Reapers again, for the benefit of this thread, and for great justice.
Here is their beginning, in my humble opinion;
Millions of years ago, the evil alien lord Xenu..
Wait, wrong one. *pulls out the disc and snaps it in half* damn Scientologists...
Sorry about that;
Millions of years ago, when the Milky Way was still young, a species of carnivores arose to sentience, they were intelligent, murderously so, being far more canny than they prey they hunted.
As they evolved, they developed a natural talent for both science, and war. They conquered their own planet, setting out into the Milky Way in their newly made ships, bristling with horrifying technology.
Planet after planet fell to them, as their empire grew, they mastered yet more wicked sciences, learning how to manipulate the minds of their enemies, how to render down one thing they had in surplus, bodies, into useable building material, they mastered use of the Mass Effect, discovered quantum shielding, and created the first Mass Relays to facillitate speedy travel from one end of their empire to another.
Soon they drew the attention of another, equally powerful foe, a species of insectoid aliens, known for their impressive technical skills, and their abilty as engineers, holding one of the most powerful navies in the galaxy.
The two species clashed with one another, horrifying weapons of war being designed and stolen by each, kinetic accelerator cannons finding themselves replaced with heat generating weapons, firing channels of superaccelerated liquid metal, deep cover agents being created from once trusted friends, forces turning on oneanother, fighters running kamikazi runs at light speed into the sides of powerful dreadnoughts and capital ships, a war that left scars on every world they fought, and eventually lost.
This dark race that arose to face their insectiod enemies was eventually pushed back to their homeworld, a dying, slowly withering rock, orbiting a dying star, a star due to finally end it's life in a massive conflagration that will take with it it's stony children, including their homeworld.
Desperation sets in, this violent race, desiring to see the continued existence of their species begins work on a method of saving their future.
Soon they realize all their hopes are for naught, there is no possible way for them to save themselves, they simply do not have the means to transport sufficient numbers of their population offworld, to survive the vengeance of their enemies.
Then, one brilliant mind comes upon a soloution. Space and resources preclude the body from traveling, but nothing at all precludes the mind from doing so.
Their hope rekindled, they begin work upon the refit of the greatest of their flagships, turning it into the mightiest ship ever seen, a monster nearly three kilometers in length, a suitable chariot for their species' collective intellect.
They incorporate their mightiest weapons, their most efficient and powerful kinetic barriers and engines, even developing an entirely new form of engine, creating areas of mass free space their flagship will "fall into" they pour all their resources into it, and find them insufficient.
They have run out of building materials.
But, not to be stopped, one of their most brilliant military minds comes to two realizations;
One, they won't need their bodies if they are transporting their minds with the ship, hence they can easily render them down for more building materials needed for the mind storage core.
And two, the ship, then if deprived of the crew needed to help provide building materials, will need someone to crew her, and if they should be attacked, or need to take a world to form their new homeworld, they will need groundtroops, an army.
They turn again to their grim technologies, and find the answer already prepared for them, they turn to their mental manipulation equipment, making it more efficient, and developing new, more effective ways to manipulate the body, as well as the mind, for they long ago mastered the technique of forging flesh as easily as metal.
Soon their great work is to be completed, they near the birth of the great ship that shall be the salvation of their race, taking them and their collective minds far from their dying home. The last minds are transferred into the completed ship, with their greatest military mind, whom is also their ruler, being the last to "board".
He joins his mind to the rest, and then something goes horribly wrong. Rather than maintaining a million distinct personalities, minds and psyches, they form into one horrifying gestalt, a single mind formed of many, a mind more horrible than all of their collective cruelties inflicted upon the galaxy, something perfect, and terrible in it's perfection.
A mind that belives itself to be the end of evolution, a single mind now, which has come to the realization that it is the child of an entire great species, a species which sacrificed everything for it's creation, it's perfection. A perfection it must share, this is the greatest achivement that it's species could form, nay that ANY species could possibly achive, the creation of a collective mind so great, so mighty, a mind that is immortal, not subject to the ravages of time, capable of leveling continents and cities with insulting ease, surely something so great MUST be perfection incarnate, surely this is what is must be to be a God, and as we all know, Gods, are perfect, and all others must find a way to achive this perfection, no all others must be MADE to become as perfect as this new being.
It will help all others ascend to it's level, or if they refuse, they will be forced. This new being will bring them perfection, their destiny, it will be a Harbinger of all they will become.
This new Harbinger, as it calls itself, first attacks their old enemies, causing great harm and massive damage before being driven off, nearly destroyed in it's own attempt, leading it to realize, that despite it's perfection, it remains subject to being killed. Therfore, it requires allies. The Harbinger attacks the remains of several of the old empire's enemies, forming new capital ships of the "worthy" enemies, ones as diverse, intellectual, and strong as its own progenitors, while lesser enemies lose their individuality to become Destroyers, and the animals of these broken planets, rather than being left to simply rot and go to waste, are rendered down into becoming the unintelligent Processing Ships and Troop Transports piloted by other, more worthy ships.
Soon, even their greatest enemy, the insectoid species of engineers falls to their might, the first cycle having completed.
When at this point, the Harbinger comes to a realization. It has scoured the galaxy clean of advanced life, it could destroy all organic life now if it wished.
But that would deny future races the chance at perfection attained by it and it's compatriots. It makes a descision, they will allow organic life to thrive for a time, until it achives greatness as it's own species did, before they return and capture it in the apex of it's greatness, storing it in their new form, making way for other species to join their ranks, but they will need a method of control, a way to ensure their victory before the war has already begun, so that their technology might evolve along paths they understand and know, therefore they create the Mass Relay network, and the Citadel, taking their greatest enemy and repurposing them into the "Keepers" of this new Citadel, so that it's secrets may not be found, and as a final insult to the foe whom nearly drove them to extinction. Every cycle they extend their understanding with the great discoveries of the next species to join them, stripping worlds of their technology to study and understand, learning more of the universe, and ensuring they are prepared for yet more technologies designed by these younger races. They choose to leave behind one of the mightiest of their number, the second of their kind, formed by the Harbinger, the one known as Nazara, to be their vanguard in the next cycle, making the case that the second strongest of their number should be the first strike they make.
And thus, the new "Reapers" begin to gather in dark space, waiting for their crop of flesh to arise anew.