Yes, if the final assault would have succeeded and the Emperor would have been destroyed, Horus would have risen to power. He would lead humanity for some time, but after that humanity would soon destroy itself and with it Chaos (because the Chaos Gods mainly gain their power from humans).
So Horus wins in Horus Heresy=Chaos would have died.
Horus loses=Humanity will live on for many millenia until Chaos becomes too strong and takes over everything.
That's the scenarios the Cabal (a gathering of aliens) presented Alpharius Omegon in order to make him take the side of Chaos (while really being a loyalist) and help destroy the Emperor, in order to destroy chaos later.