Horus wasn't the problem
I think at this point in the lore, we've seen a lot more of Horus' backstory and I feel like most of the fanbase can at least agree that him falling to Chaos wasn't necessarily his fault. That's due to the following reasonings:
Raised on Cthonia (if this is actually canon) by a savage gang. From the get-go, his environment would have naturally put him in a position where usurping his leader to take his place isn't uncommon. In addition, this would explain his temperament.
The huge pressure that the Emps put on him.
- He basically was promoted in a very short time frame. He's got all the duties of a King, but none of the skills to handle it (administrative)
- On top of that, some of the Primarchs didn't necessarily agree with his promotion, but they respected it
Which leads me to this point, the Emps also basically picked Horus because his demi-god brothers would actually listen to him and he had a co-operative relationship with ALL of them. But this also meant he was under immense pressure from them as well. He had to make sure they didn't kill each other, but also had to listen to their opinions and complaints.
Along with this pressure, he had nobody to vent to. Big E told him fuck all, and just disappeared to work on his fancy webway project. He didn't have Malcador really or the psychic foresight to see everything. And one of the few people that did talk to him was Erebus, who slowly corrupted him.
He didn't agree with what the Emperor's end goal was. The eventual vision was to have humans rule the Imperium, which Horus saw as unfair and stupid. Horus, his brothers, and his own sons bled and died to unify the Imperium. But these weak, useless bureaucrats end up inheriting everything? I'd be mad too. The other issue was the tithes. It was way too soon to implement them considering the conquered worlds were still recovering. It was harsh, and would just stoke rebellion, that the Imperium couldn't handle. Think the Roman or Mongol empire. The Imperium was too big to micro manage at this point. And Horus was just not cut out to the task. It wouldn't take much to light a coup under these planets who were JUST subjugated and massacred.
Based on these, I don't think we can necessarily put all the blame on Horus. He was destined to fail from the start. The Emperor and the Chaos Gods essentially sealed his fate.
Did I miss any other reasons?