How did a man who loved the Emperor as his father and his only God betray the Emperor?
What force turned love into hate?
That Colchis had pockets of chaos worship is known, that Lorgar struck down these cults before joining the Imperium is also known.
What is less known is what happened to some of these ex cultists. We turn our attention to one in particular. Erebus.
Once a slave to the ruinous powers, always a slave.
And so it was with Erebus, though he turned his cloak and claimed fealty to Lorgar, this was a deception.
The Emperor's rebuke of Lorgar was a trigger not for betrayal, but for soul searching. His faith shattered he was vulnerable to the first Idealogue that showed up, and to his misfortune one was already in his retinue.
And so fell Lorgar, to depths he never returned from.
His people would say later that they did as their ethics dictated. The discovered the answer to Mans first mystery. What happens after you die?
They found Gods and an afterlife to exist, and though horrified, could not turn away from this basic truth. And once damned, they appeared reconciled to there being no hope of redemption.