Unfortunately as a Raptor Crixus doesn't really belong to any Legion. The Raptors broke off into their own cults during the Heresy and so although he still pays homage to his original Legion Crixus's true loyalties are really only to the search for pleasure and fulfilling his need to cause harm to the helpless.

For the Night Lords dealing with these wayward deviants is a matter of necessity. Sometimes you need the skills of the airborne parasites to aid you in your victories, even if it means having to purify yourselves for several weeks afterwards.

On another note, Crixus tends to look down on the Night Lords as they stand now, seeing only usurpers to the true Legion, those who were born on Terra and raised up by the Night Haunter himself. Having fought at his Primarch's side he feels sullied to have to work with the ill-spawned children of Nostramo and Tsagualsa who currently pollute the ranks of a once great and powerful force.