That was my fault I'm working on correcting the mistake now it's Soul Drinkers. We have a chaos and a Soul Drinkers chainsword and I didn't look at the image before posting. My fault :( going to have to give my life to the emperor due to my failure :(!
NO apologies necessary, I actually thought it was a some sort of sympathetic Chaos 'for freedom', and wait, aren't Soul Drinkers Chaos? I remember the first book, it was all pretty good, the SDs seemed like loyal Imperials who were trying to do the right thing, till one of them turned into a spider. At that point how are you not Chaos marines?
From what I read they consider themselves loyal to the emperor. Not loyal to the imperium. They are very susceptible to being touched by Chaos but are still considered a Space Marine Chapter. I'm sure Inquisitors have a hayday whenever the Soul Drinkers are in close vicinity :D.