So I'm pretty up on the Thousand Sons fluff, but it's still not quite clear to me who is working to carry out the will of Tzeentch, and who is trying to thwart him? Magnus, while harboring a lot of ill will toward the Space Wolves, doesn't seem to be particularly antagonistic toward his father the Emperor or the rest of the Imperium. In the Black Library books, he often resists the will of Tzeentch, too.
Similarly, Ahriman's Rubric spell was meant to cure his legion of Tzeentch's "gifts" of mutation. In the Black Library books, he also seems to resist the temptations/schemes of Tzeentch.
Yet Tzeentch was somehow named the patron god of the Thousand Sons somewhere in all of that background story, and Ahriman and Magnus are also currently at odds with each other because of the Rubric. It's a little confusing as to which is the traitor who should be embracing Tzeentch, and which is the loyalist-turned-reluctant-traitor working to thwart Tzeentch's will?