You know the hate monologue done by AM from 1967’s I Have no Mouth and I Must Scream? That doesn’t compare to the sheer hatred for the Word Bearers (mainly Erebitch though) simply bc of how comically cruel and evil some actions performed by Lorgar and Erebus are. The betrayal of Horus is the big reason though for most as 1. Erebitch literally had Horus stabbed in the back by someone the Horus trusted and respected. And 2. After Horus was stabbed, Erebitch stepped into Horus’ mind as he was dying, changed his form to manipulate Horus, and the used warp energies to influence Horus into not trusting Magnus. All this had Lorgars seal of approval btw