I'm fairly sure Precursors were against violence, seeing as they were overthrown by a less advanced race of their own creation. Wouldn't that make them redundant in a war?
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They weren't pacifists, they were dying from boredom.
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The precursors were so bored with their lives that they put the forerunners through a series of tests to see if they were ready to take their place and uphold the mantle. When the forerunners failed those tests the precursors parished and then came the flood.