>>96889828
The personality flaw Erebus exploited was that Horus was mad he wasn't as informed of his father's plans as he felt he should have been, that the Astartes weren't going to have the role in society after the crusade that he felt they should, despite everything he'd been given he lost his shit because he was being treated as a servant like everyone else, not an equal, not even as high a servant as Malcador. Without that resentment, Erebus would have failed.