There is no evidence that they are actively "furthering the ways of chaos" to topple the imperium like the word bearers and black legion. In fact non of the god specific legions do!
Think about the death guard cause some plagues, nasty but hasn't got the aim to kill everyone
World eater kill maim burn, mehh, cities can be rebuilt and repopulated
ECs just go and get high, not toppling an empire there
Destroying the imperium really doesn't matter
Also theres no evidence that it was Horus, more than likely it was Valdor. And Magnus likes horus and tried to save him and is very close to Lorgar, so it makes sense to side with those two than just wait to die
By embracing Chaos, revelling in it and spreading its corruption, the 'God specific Legions' are actively furthering the ways of Chaos. Slaneeshi cults? Nurgle-devoted cults? The countless ones you here about from almost every world in the Warhammer 40k Universe? That pretty much is furthering the spread of Chaos.
Destroying the Imperium matters very much to them. Despite their constant internecine wars the Traitor Legions all hate the Imperium immensely, it borders on obsession. Infact, it is an obsession for them.
There is actually more evidence right now for it being Horus who manipulated the Wolves, since it was actually stated in the earlier Heresy novels, than it being Valdor who, quite frankly, I haven't heard one convincing argument for.
Magnus may well have liked Horus and Lorgar, but after learning that Horus and Lorgar were about to bring the Galaxy to the brink of annihilation, I think he may have reconsidered. And I'm pretty sure that Magnus could do more than simply wait to die.