Oh dear.^^^Loud
A tricky question to answer and certainly not a particularly safe one given the current political climate, especially if you are posting this from imperial space - I hope you've anonymised your astropathic trail!
First of all, I have to address your use of terminology. Why do you see Warmaster Horus as a heretic? Nor should we call the Emperor the "God-Emperor" at this point. Do keep in mind that the Imperial Cult had yet to be established and in fact would not be officially recognised as an official body of the Imperium for many centuries after the Great War. More importantly, the Emperor at this point was promoting the so-called Imperial Truth, the view that were no gods or demons. Luckily, in the grim darkness of the fortieth millenium we know better - the gods are real and to deny their power is just foolish.
Rather than seeing the Great War as a war between a "heretic" and the "God-Emperor", I see it as a continuation of the Great Crusade and the result of internal Crusade politics between different imperial factions. Note for instance the enmity between the various Primarchs, the tension between the newly founded civilian administration and the military which fought so hard to build the Imperium, and of course the wasteful project of the Emperor to build a connection to the Eldar Webway within his palace - he clearly wasn't aware of the dire financial crisis his new Imperium faced! In this troubled time, the Warmaster saw that the Imperium could not last if nothing changed. To save the Imperium, he must let the galaxy burn. A tough choice I'll admit, but Horus was anything but an ordinary man. He was the mightiest of the Primarchs and well-versed in both warfare and politics. Though he loved the Emperor, he also recognised that the Emperor was an insurmountable barrier to reform. Horus naturally made the necessary sacrifices and recruited new, more powerful allies to supplement his Astartes Legions and led his forces to the Isstvan system, where the fates of many a legion were decided, as you should know if you had taken a look at the Inquisitorial archives already (if you haven't, remember that Inquisitors take censoring their inglorious history rather seriously, I recommend making a few pacts with daemons first before venturing into their libraries, just to be safe).
At this point, it is also worth noting that you shouldn't take seriously the scandalous claim that a Word Bearer Chaplain, Erebus, had deliberately poisoned the Warmaster and then introduced him to the Warp in this fashion, which then allegedly "corrupted" him. I think there is no ground to believe that. The Word Bearers were and are the proudest champions of our beliefs, they would never stoop so low as to use trickery to bring about the greatest war in history. This is just another lie produced by the Black Library (a coven of insidious Eldar if you didn't know) and their allies in the Inquisition, so don't believe a word of it. This might also contradict what you have heard from the official imperial histories, but note that in the aftermath of the Great War history was literally rewritten by the survivors on Terra to paint Horus as the villain, when if anything he was the hero. As I said above, there is also a great deal of censorship going on even today. For the truth, I highly recommend you to get a hold of the following grimoires - they might be a bit hard to acquire in imperial space, especially as some can only be read within the Eye of Terror (a misnomer, I assure you), but they are really worth the sacrifices!
For a short fun read, try the Lectitio Divinitatus, a text from a cult that worshiped the Emperor and published just before the War began (ironically it was penned by the Primarch Lorgar), boy were they crazy! It is an authentic source from this period though, so it is worth taking a look, even if they were wrong about everything.
Because he was mortally wounded by chaos and the only way to save his own life was to submit to the dark forces of the void. This led to being corrupted by the chaos gods, and the beginning of the Horus Heresy. The God-Emperor was busy with a secret project, attempting to open re-open the Eldar gateways, and did not see the betrayal until it was too late. The Primarchs were divided and a civil war broke out in the Impurium of mankind. Sanguines (Blood Angels Primarch) was killed fighting Horus but weakened Horus's armor. The emperor faced Horus and defeated him in battle. The fight almost killed the Emperor and left him bound eternally to the Golden Throne.
I would like to start by correcting the notion that Horus was a heretic. He was not. Infact he was among the greatest of the prophets sent to mankind by Grandfather Nurgle and his brothers to lead them out of the xenophobic propaganda being put out by the False Emperor.
The terrible power of the False Emperor had been so great that Grandfather Nurgle and his three brothers (even the youngling Slaanesh) had to work together to pierce the veil and let Horus see the light. The wisest among all mankind, Horus was chosen because he had the greatest chance at success in breaking the iron grip the Emperor had on the humans. However he failed. Barely. He sacrificed his own life, valiantly fighting against the False emperor himself, and managed to grievously wound the False Emperor. With his body weakened, the magics the False Emperor had used to hide his corpse like body withered away and all the people could see him for what he truly was, a Corpse Emperor.
Even though Horus is dead, his legacy lives on. The legions, assisted by the warp entities, still try to help the people see the light. But the indoctrination is too deep and it will take a long time before the work that Horus has started will see fruition.