Those familiar with the Changer of Ways know that he is the most cunning of the Powers that influence the Galaxy.
As with the rest of the Chaos deities, Tzeentch greatle envied the Emperor's children--the Primarchs. Magnus especially must have been his target from day one, given his immense psychic abilities. How to win him over, though? Tzeentch simply could not influence a being possessing Magnus' power directly--not outside the Immaterium. And Magnus countered the mutations Tzeentch inflicted on his Sons; it cost him an eye the first time, but he knew how to defeat such an attack now... unless he could be distracted somehow.
So, cunning Tzeentch hatched a scheme within the greater scheme his brethren were orchestrating. Knowing that Magnus would never betray his father and could not be coerced to do so by the means the other Primarchs were, Tzeentch turned to other targets.
The Changer of Ways corrupted the blunt, barbaric, and decidedly non-cerebral Leman Russ. He warped his being as surely as he did the Gene-seed his Space Wolves depended on. Amazingly, no one identified Tzeentch's influence as a source for the all-too obvious lycanthropic mutations the sons of Fenris suffered from. He twisted their minds as surely as he twisted their bodies, making remarkable hypocrites of them: they frothed, raged, and hurled their offal when around sorcerers like the Thousand Sons, even as they jealously guarded and zealously preserved the petty tricks of rune-carving and bone-casting Tzeentch bestowed upon them.
It was easy, thus, to use a lesser pawn--the Space Wolves and Russ--in order to gain the greater piece: the Thousand Sons and Magnus. Having Horus change Russ' orders from a simple capture to a genocide was easy enough, especially given Russ' masochistic and/or blind willingness to happily embark on actions that would obviously lead to great disaster for his father's Imperium.
Having to focus his energies and powers to defeat another Primarch handily distracted Magnus, enabling Tzeentch to back-door his changing ways into Prospero, which led to the Thousand Sons re-mutating at the most crucial juncture. Abandoned by the Imperium, faced with the spectre of losing all his Sons... Magnus finally had to accept Tzeentch.
Best of all, Tzeentch maintains the best sleeper agents within the Imperium. He manipulated his childishly maverick pawns into throwing a tantrum over the re-structuring of the Codex Astartes, which forced Rogal Dorn to also take a stance of independence, for no other reason than pride--how could he bow down when the least useful Primarch in the Imperium wouldn't? This of course led to the Imperial Fists being decimated in the Iron Cage incident... only ONE example of Tzeentch's use of the Space Wolves to de-stabilize his enemies for his own amusement.
Despite their obvious touch of Chaos, Space Wolves remain greatly respected to this day--Logan Grimnar is even counted as one of the most beloved heroes of Man!
Note: the above was of course delivered with a heavy dose of sarcasm!