And likely why the Space Wolves are still loyal unlike the Thousand Sons.I can never see Khorn as sneaky/plotting.
But I digress, the Wolfdna blending of the early fenrisian colonists was to better stand live on their death world by drawing on the strenght of the wolf. But chaos enlarges and magnifies negative character traits, manifesting all that in those that gave in to their inner beast. Letting it be fully unchained. Something which the space wolves fights with on a constant basis. In the first ragnar blackmane novel, theres lots of good sequences of how he is squaring off against the beast that have been roused inside him and how he learns to live with it and not succumbing to it or embracing it to turn into a feral, mindless monster only driven by its urges and hungers.
We dont know how many Space wolves that fails that test during initiation, but even then early success is no guarantee as the initiate may devolve into a ravening wulfen. Only the barest few managing to keep control of themselves like Redmaw. So many similarities with the problems had by the Blood Angels, and Id presume the Thousand Sons as well if we knew more about how the flesh change would occur for them. Only the Space Wolves seems to have a much better grasp with dealing with their internal monster.
Edit: Hrm, you know. Perhaps its the culture of the space wolves that makes them much better suited to deal with their Primarch's legacy that may may not have been caused by the dark gods. But look at the Space Wolves, young uns are formed into Blood Claw packs where they are taught how to deal with their inner beast and their aggression. Whereas compared to the Blood Angels, they would rather pretend their gene curse not existing, every struggle a solitary affair until the mind of an unfortunate snaps and he becomes taken by the blood hunger and eventually ending up in the Death Company, or axed by Astorath the Grim. Making their curse a constant source of fear none of them wants to talk about. But we know that the curse is beatable, exceptional Blood Angels have forced it back down. Though it leaves one wondering if more would succeed if they would more openly face their problem instead of considering it taboo until it raises its head.