Okay hear me out.

The Forerunners talked shit about Humanity being arrogant, when the Forerunners themselves were arrogant enough to kill off the precursors to try to claim the Mantle of Responsibility that was meant for Humanity. The forerunners gave the Precursors cause to turn themselves into powder in order for them to come back eventually, but the powder was corrupted by what was likely a combination of radiation and emotion (weird ik), which turned into the flood. The species in control of the Mantle of Responsibility is meant to take great care of the universe, but the Forerunners caused the flood, so they had to create this superweapon to destroy all sentient life to stop the flood, and killing what they were supposed to protect. Fast forward to the Human-Covenant war. The Covenant are praising the same motherfuckers that caused the flood to exist in the first place, and are trying to get salvation.