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So I've never read them HH books but one thing that's really bothered me is Horus' choice of how he'd start his rebellion.
So with the total trust of the Emperor and command of the Imperial military, he trucks off to some politically insignificant planet and blows it up, along with everyone in 4 legions who would have supported the Emperor in the event of an insurrection (most of them anyway).
Then he hunkers down on a planet in the same system and waits for Imperial justice. Then he trusts the 4 legions of the second wave of imperial attackers enough that they form an integral part of his battle plan and not one man in those four legions warns the other three legions about what the second wave is up to. Thus, the Dropsite Massacre goes down like clockwork.
Then Horus and company start trekking all the way to Terra, relying on the masses to give him popular support the whole way. Right?

So if his plan was to throw down the Emperor, why didn't Horus start his rebellion by killing the Emperor? He was organized enough to accurately predict who would support him without his rank-and-file getting smart enough to warn anyone before the Heresy began.
If I was Horus I'd take the Sons of Horus, and the Death Guard, Emperor's Children and World Eaters put down the rebellion on Isstvan, then leave the loyalists within those four legions behind on Isstvan to "garrison" it. Then I'd return to Terra, saying "I want to start a new campaign. I'm rallying the legions at Terra" or "I wanna have a ceremony parade to show the unity of the legions under my new command to show that this warmaster system works" or "this is an emergency, something's wrong with my men's genes. I need the Emperor to take a look at them and tell me what it is." In any case, with the Emperor's full trust, I'd get myself close to him, get him in private and Angron and I would kill him while my legions occupy Terra. Then I'd either blame one of the Imperium's many enemies for the Emperor's death (if it was possible to) or say "yeah, i did it." In either case, I'd explain that I was second in command and the natural heir to the Emperor, adding that due to its size and the importance of the military, the Imperium should be led by the Warmaster, adding that the Emperor had grown lax by retreating to Terra and relying on his Warmaster to keep the peace.
Then I'd start using my influence to sway worlds to worship chaos.

So is there a really good reason why Horus couldn't knock off the Emperor before beginning his rebellion? Is there some little-known edict in the HH books that makes the above scenario impossible?

Because Grimdark.

But really, because Horus didn't want to win by sucker punch. He wanted to dominate. And carve his way to the top dog.

Pride and ambition. His best and worst character traits.

Plus the Emperor was busy in his lab, receiving no visitors, holding no audiences, and determined to not be disturbed until his great work was done.

Because that would have made for a very short story?

 pretre wrote:
Because that would have made for a very short story?

This.

Because then what? He would've had 9 full Legions coming down on his head, including the ridiculously massive Ultramarines force, with himself on Terra trapped between them all. The loyal Legions wouldn't have simply said 'Huh, the Emperor's been murdered, all hail Horus!" Horus would find himself in a galaxy spanning war of attrition with a numerically inferior force. The way he did it allowed him to almost destroy 4 full Legions before the war had even begun, and he seemed to have the Heresy in the bag up until the 1v1 fight with the Emperor.

Also Dont forget the Adeptus Custodes

he should have started his own empire

If he would have killed the Emperor at the start they wouldnt have had a common goal. The only reason the traitor legions worked together was to end the rule of the Emperor. This way if Horus had won there would be 9 legions remaining, with the real possibility of infighting remaining. Killing him at the start would risk triggering a 18 legion free for all. Grudges to settle (iron warrior/imperial fists, word bearers/ultramarines), switching of loyalties (the regret of Cruze, Alpha legion). The way he did it made it possible to eliminate his obvious enemies first and then dealing with the others instead of risking a much more fractured civil war.

 Disciple of Fate wrote:
If he would have killed the Emperor at the start they wouldnt have had a common goal. The only reason the traitor legions worked together was to end the rule of the Emperor. This way if Horus had won there would be 9 legions remaining, with the real possibility of infighting remaining. Killing him at the start would risk triggering a 18 legion free for all. Grudges to settle (iron warrior/imperial fists, word bearers/ultramarines), switching of loyalties (the regret of Cruze, Alpha legion). The way he did it made it possible to eliminate his obvious enemies first and then dealing with the others instead of risking a much more fractured civil war.

This ^

Also, for all of Horus' faults he still had some honor, at one point both he and the Emperor agree to stop using assassins against eachother. I think he wanted to win by domination, and not some underhanded coup.

At the point of the Dropsite Massacre, if Horus had faced the Emperor head on, he'd have been wiped out.

He spends years, decades, raiding and striking, building his forces, harassing the Imperium, stockpiling weapons and resources, raising support, turning others to his cause.

If he'd faced the Emperor in person, then, he'd have been killed in an instant. It was only when the Emperor tried to redeem him, left his guard down AND Horus was imbued with the power of all four Chaos gods, having wasted most of the lives of his legions and the entire rebellion to get to the Emperor's Palace, that he mortally wounded him.

And, of course, pride on all counts. Everyone knew that Horus didn't want to win or lose in a distant arm of the galaxy.

As Dorn himself was told, "this is where it will all be settled, on Terra.".

Also don't forget that the istvaan 3 incident was symbolic. To free the legions from loyalists within. The utter annihilation was supposed to get all faces in the same direction. the purged legions were free and had thrown off all connections with the imperium the emperor stood for. You can't fight a rebellion when there are elements in your legion still loyal to the emperor. It's the same as groups within the taliban saying we love america and the west. How serious would you take the taliban then.

Because that isn't as epic as massacreing loyalists in your own army then massacreing other loyalist legions by showing your other allies in the rebellion i guess

As people have said, even if Horus had managed to take the Emperor out on Terra, it's doubtful that he would have made it out. As people have said, he'd be trapped away from his powerbase, with a bunch of angry Custodes and Astartes between him and everything else. And, he would have had little support. It's only his long campaign of building support and showing that he had a chance of winning against the imperial forces that actually allowed him to build up the support of a lot of people.

he didn't really think it through - if i ever turn to chaos i'll have to remember to ask the gods more questions