The Emperor is an idiot. His complete detachment from his own humanity is what fucked up everything in the Horus Heresy.
Not giving more attention to his kids - who were, themselves, peerless monstruosities that ended up feeling alienated or alone in the universe - was the first mistake.
Harboring competition without reeling them in was his second mistake. Some of them took it way, way too far to compete for his attention.
His third mistake was to never tell them about Chaos. Holy Terra, did he fuck up here! This was especially true with both Lorgar, Magnus and Fulgrim. Had they known about Chaos, they may not have fallen to it so readily. Magnus should also have received extensive training and knowledge about the warp.
The Emprah also should never have retired from the crusade to build the Golden Toilet without telling his sons exactly what the fuck he was doing. Had he done so, they may not have resented him for abandoning the crusade, and Magnus, especially, wouldn't have fucked up his project with his communication attempt. Maybe he would have tried to connect to Malcador, instead (if there was even a Heresy to begin with) to warn him if anyone was going full heresy.
He should have dealt with Lorgar with a lot more tract. Humiliating him and his legion only made him vulnerable to the heretical whispers of Kor Phareon and Erebus.
The whole Horus Heresy is a result of the Emperor forgetting his own sons were much more human than he, himself, could ever hope to be.