Im kind of confused in the lore here, so Precursor want to destroy humans why..?
The Precursors (timeless, incorporeal, godlike beings) have a belief in the Mantle, that every race has the right to the chance to attain their maximum potential.
The Precursors created a specific species to give the Mantle to, so the Precursors wouldn't have to micro-manage life in the galaxy forever.
The race the Precursors made eventually split into two. The first stayed around their homeworld, and delved into genetic manipulation and stellar engineering. They took the traits they found most admirable in other species and incorporated them into their own genetics. They also accidentally destroyed their homeworld when they made their sun go supernova. The second race descended from the Precursors chosen successors went to the far side of the galaxy, and the first faction eventually forgot about them. This second race developed as the Precursors intended them to.
The first race descended from the Precursors' chosen successors are the Forerunners.
The second race descended from the Precursors' chosen successors are the Humans.
The Precursors chose to keep their promise exclusively through the human descendants of the original race, because the Forerunners proved to be unworthy of the Mantle. However, the Precursors couldn't exterminate the Forerunners, because the Forerunners had not achieved their maximum potential yet, so the Precursors allowed the Forerunners to slaughter the Precursors' biological avatars, and allowed the Forerunners to develop until they did peak, at the end of the Forerunner/Flood war, at which time the Precursors stopped holding back, and the Flood destroyed the Greater Ark with the last remnants of Forerunner civilization on it, leaving the Lesser Ark with humanity and a variety of other races to repopulate the galaxy after the Halos fired.
The Precursors aren't coming back, because their plan to establish their chosen successors is nearing completion, and the point of that was so the Precursors wouldn't have to keep intervening.
I don't expect the Flood to get loose in a major way again, not because they can't, but because the Flood ultimately serves the Precursors' goals, acting as villains to unite the galaxy when needed, and if the Flood were to be unleashed, there wouldn't be any plausible means of stopping it that wouldn't be it obviously taking the fall.