Actually the emperor already declared the Thousand Sons *almost* heretics
as in "pending inspection" after Magnus used chaosy spells to contact the Emperor
Russ was ordered to ESCORT Magnus to the Emperor but Horus persuaded the old hot headed space wolf to attack Magnus instead, as he was a disgrace and a threat to the emperor or some other trivial reason
And magnus was never trully 100% pro chaos.
Not even after he became a demon prince.
All he cared about after he ran away was taking care of his legion.
He almost died of a heart attack when he saw what Ahriman had done to his legion
