The sins to which Gravemind may attribute to us is the Precursor's belief that all of their creations should possess individualism and free will at their very core. This backfired against them when Forerunners deemed themselves the rightful inheritors of the Mantle. The Precursors faced mass genocide when they decided to instead place that burden on the shoulders of humanity instead of their sister race, the Forerunners. Then, the Forerunner waged war on the Precursor's creations throughout the Milky Way, including humanity, to firmly establish themselves as possessing the Mantle.But the statement is still a little odd considering it's directed at a Human. It would have been more appropriate if it had been directed at a Forerunner. The Gravemind seemed pleased with their revenge over the Forerunners and while the Primordial deemed it best for all to be integrated into the Flood, Humanity was going to be given the chance of proving themselves worthy of the Mantle.
Actually, I think Halopedia can sum this up a little better than I can:
Quote:The Gravemind often touts to others that the Flood represent peace and salvation. At the same time, however, it is very much aware of the pain and suffering its victims are subjected to. It sees this as fair retribution for the Forerunners' crimes against its Precursor ancestors. According to the Gravemind, the Flood, down to its very nature and the suffering it inflicts, serves as an appropriate reflection and reminder of the pain that the Precursors ultimately brought on themselves by giving their creations free will; it considers itself a "monument" to the sins of other sentient beings. It regards the Flood as a continuation of the Precursors' desire to create life, but unlike before, the life they bring forth would no longer possess free will or individuality. Embittered by the Forerunners' rebellion, the Gravemind saw the Flood as a means to unite all life into one, so that no conflict or competition would ever arise again and that all life would coexist peacefully as one.
"Child of my enemy, why have you come? I offer no forgiveness, a father's sins, passed to his son." ~The Gravemind
I had something to say in addition to this but it’s slipped my mind.