After being explicitly forbidden to pursue their vile research, which they ignored. Obviously, proving themselves untrustworthy and turning themselves into abominations is a good cause for being exiled.
You drank the kool-aid.And you probably should read the race description a bit if you truely believe they are adherents to any god. They do not worship the Void, Void Lords or Old Gods, they are not cultists in any way... on the contrary. They just use the Void as a tool, a weapon like any other.
NOT Succumbing to it's power and NOT obeying the whispers they hear is their entire deal.
Oh, sure, they probably believe that they're unaffected. They're wrong.