It's been hinted at before, but I'm a sucker for a concise submission.
Kyle Maurer (Rykaziel@bu.edu) writes:
Of all the statements that Gravemind makes, there is one that continually surfaces in my mind.
Gravemind:: "I? I am a monument to all your sins."
I believe this statement reveals much about Graveminds origin and the fate of the Forerunner. However, the following analysis of this quote assumes that at least one thing is true :: That the Master Chief and Cortana resemble the Forerunner, and as far as the monitors of the installations and Gravemind are concerned, Humans (especially Master Chief) are regarded as Forerunner or at least meet certain criteria that would make Gravemind and the monitors assume Humans are connected in some way.
With that out of the way, let's examine the quote. The two most striking images are monument and sins. Trying to remove extreme religious symbolism, we can imply that sins means a wrongdoing, an act that goes against a natural law. A monument, is a testament, or a physical, timeless interpretation or product of an event. So to clarify Gravemind's statement, he is the product, or embodiment, of Forerunner wrongdoing.
My hypothesis, is that the Forerunner began to experiment with genetics, and got past the point of cloning, until they tried to create a new life altogether. Their intent may have been wholly moral, and wholly good, but they emerged was something terrible. As an example, the Forerunner may have given the Flood the incredible regenerating capabilities it has, thinking maybe the Flood could be used for rejuvenating an endangered species. However, whether intentional, an error in genetic code, or as nature's retaliation for trying to do what only nature can, it also had it's virus-like characteristics. The Flood, the Forerunner creation, became their demise.
Because of the Forerunner sin against nature, Gravemind, an old, old Flood form, may have been one of the earlier forms from which the rest of the Flood came from, or simply one of many very old advanced forms. He is the image, the terrible product of Forerunner arrogance, and he endures as a monument of the Forerunner's colossal sin against nature.
As for Gravemind's intentions, that remains to be seen.