"Chaos cults are insidious, and they can spring up practically anywhere, sucking in the basest and most degenerate specimens of humanity. Their greatest threat, however, is that as they grow they bring in more recruits who may remain unaware at first of what precisely it is they're joining; they may think it's a street gang, a political movement, or a social club for those with a particular kind of sexual deviancy.
Only as they grow more corrupted by their patron power do they realise the full extent of what they're a part of, and by that time the lies and illusions are too strong.They've become damned and they don't even care."
Krems confirmed chaos cultists. Purge with fire. Exterminatus is the only way to be sure.