So far the Horus Heresy has had:
-Erda, the 'mother' of the Primarchs, who after helping the Emperor for thousands of years conquer untold number of civilizations/people decided that reuniting Humanity using the Primarchs they made as warlords was going too far. She then proceeds to throw her children into the Warp to 'save' them from the Emperor.
-Astarte, the woman who now is the person who helped make the Astartes which end up named after her, betrays the Emperor in a stupid rebellion that is easily put down. Despite being a traitor the legions of Space Marines would be named in her honor for...reasons?
-Selenar Gene cult witches who possess better info on gene seed that the Emperor or Astarte did somehow and try to keep it from the Emperor. Why? Because he is a man, and men only destroy things. They will then go on to side with Horus, who at that point has become the champion of Chaos, and it is only through a series of sacrifices by heroic Astartes and others that their vital intel instead gets recovered by the Imperium (in part thanks to Magnus and Alpharius helping them). Moral of story, men bad so join worse men who will cause extinction level catastrophe?
These characters introduced into the setting to strip away the Emperor's achievements all suck and are awful on a great many levels. Astarte is a redundant, pointless character. Erda changed a feat of the Chaos Gods stealing and scattering the Primarchs into Space Karen Took The Kids. And the Selenar? They exist to say "Men bad, women good" and also possess the intel that will eventually result in the creation of Primaris.
If not for incompetence on the part of their writers I would think that these characters were actually an effort to make women look bad, because now the entire Horus Heresy could have been avoided if Erda wasn't a dumbass. To save them from serving as generals in war Erda condemned Angron to slavery, Mortarion to suffering, Perturabo to a tyrant and the gaze of the Warp itself always peering down at him, Lorgar being raised by Chaos cultists, Curze not getting the help with his Psyker powers he needed and going insane, Alpharius to being mentally scarred by a Daemon nearly killing him in his incubation pod, Omegon to over a century of solitude from the Imperium, the Alpha-Omega twins possibly having been made twins with Omegon's theory that their being tossed through the warp was what split their soul in two, and all the resulting suffering of all these traumas and issues that would play out on a galactic scale.
Seriously, these are among the worst characters in all of 40k, and 40k has a lot of competition on that front.
I would like to propose that Dorn having the personality and brain of a particularly smooth stone also was due to being thrown through the Warp, because I doubt the Emperor intended to give his son autism. That seems more like an Erda feature, in which case it may be genetic and thus excusable.
