Whats crazy is that Magnus was actually one of the good guys. He actually found out Horus had fallen to Chaos and that he was about to betray the Imperium and he warned the Emperor but the Emperor didn't believe him and accused him of heresy and ordered the Space Wolves to destroy the Thousand Sons and to arrest Magnus and to bring him to Terra for trial and that was what pushed him into accepting the offer of salvation from Tzeentch.
Whats crazy is that Magnus was actually one of the good guys. He actually found out Horus had fallen to Chaos and that he was about to betray the Imperium and he warned the Emperor but the Emperor didn't believe him and accused him of heresy and ordered the Space Wolves to destroy the Thousand Sons and to arrest Magnus and to bring him to Terra for trial and that was what pushed him into accepting the offer of salvation from Tzeentch.