system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] Why weren't the Traitor Legions and their Primarchs erased from Imperium history and expunged from the records?

Surely whatever the II and XI did couldn't have been worse than causing the worst civil war in the history of mankind?
gpt
Because it's not safe to forget.  Legiones II and XI are *gone*, so the Emperor could impose damnatio memoriae without risking harm to the Imperium as a result.  The Traitor Legions are still out there, a real and present threat to the Imperium.  While ordinary civilians don't know anything about Chaos Marines (since knowledge of Chaos is in itself dangerous) it would be idiotic for the leadership of the Imperium to erase their records of their most hated enemy.  And that's not even considering the loyalist Space Marines, some of whom *remember* the Horus Heresy, and all of whom hold a grudge...

I imagine that, when and if Chaos ceases to exist, the Imperium will swiftly and happily purge their archives and consign the memories of the Traitor Legions to oblivion. Good luck with that, though.