Its all about the mantle, the forerunners were afraid that the precursors would simply ignore them, that is the sole reason of the Precursor-Forerunner war was who was going to get the mantle.
The Precursors, instead of fighting back the Forerunners, would instead show them, that the mantle does not belong to forerunners, it really dosen’t.
During the events of Silentium, its clear that Forerunners have failed the mantle, not only did they unintentionally destroy an unknown race, but they also, on the edge of existance, almost destroyed humanity (didacts actions not only almost drove humanity extinct, but even after 100,000 years he tried to do it again), they also devolved humanity, making human kind an abomination. This leads to the only logical conclusion, that the whole forerunner flood-war was just a demonstration to show the forerunners that the mantle is not theirs to take care of.
There are a couple of things that can explain the actions of the flood in the games (UEG era). The most important one is that they wanted to test humanity again !
You might ask: But they already have tested humanity ? (AH humans)
Well this is simple to answer. Ancient humanity is almost a completely different species than us, the devolvment of humanity reduced human DNA to a different level, Librarian states that during halo 4 terminals. That is also why the flood did not allow complete destruction of human race, instead only letting them devolve them. That is also why the flood even having only high charity, did not use any precursor artifacts, because humanity was weak, using precursor artifacts would completely destroy them. I am also fairly sure that not all precursor artifacts are destroyed, even tho Path kethona and milky way both got cleared by halo pulses, there must still be left artifacts in intergalactic space (perhaps greater ark ?).
I can back this theory up by the fact that in halo wars, after the forerunner ships were activated, the gravemind on installation 05 was already formed, why didnt they use them ? They had a direct chance to escape not only the shield world, but destroy all the species in the galaxy with those ships. They didnt do it because humanity is too weak for those ships, if they had used them, then the test would not be valid.
TL;DR: Forerunner-Flood war was just a demonstration, Precursors are looking for a guardian of the galaxy or maybe a race to join with ?
What do you think ?
