When MC asks who Gravemind is, he replies "I am a monument to all your sins". And in Halo 3 he says "Child of my enemy". I've been asking myself constantly what this means and I might have come up with an answer.
The "I am a monument to all your sins" might reffer to the forerunner having a civil war, and in the anger they accidently create the flood. Or they could have fought another race and done the same.
"Child of my enemy", After the forerunner accidently make the flood they want to eliminate them.
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