What does "fight for chaos" mean in the real term?
Most of the sons actions since the heresy has been seeking personal treasure/ knowledge and seeking retribution/ revenge on the space wolves for destroying Prospero
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