I myself havent read the books. But from reading everything in this topic I think i might have an Idea for why the precursors left and why the Forerunners rebelled. It may be a bit simple but it makes sense to me. I think that the Precursors, looking for a race to uphold the mantle created the flood. This makes sense to me from the quote I have been seeing "tested by floods". Whats more like a flood than a parasite that will keep growing, keep devouring mindlessly, Nothing. So if they wanted to test the Ancient Humans why do it with anything else but a parasite? I also think thats why the forerunners rebelled. They got wind of this plan believing (and it might have been) it was a test against them and decided to rebel to stop this from happening. Now after the Precursors place flood by the Human colonies, and with the outbreaks happening in their sector they decide to leave as it is too risky to stay and expose themselves to the parasite and they also want to see how the Ancient humans and forerunners handle this on their own. So the Precursors leave and the humans are left trying to stop the flood. The forerunners having (to their knowledge) kicked the precursors out of the galaxy believe that they have stopped their plan before it started (They believe the flood havent been created). Which is why the Humans controlling forerunner space angered them for reasons already posted and it goes on from there. It makes sense to me and it seems like something that would happen.