The DH are half demons, in the initial experience of the DH Illidan confirms to the character that he has a demon soul just like him. Following this line, the devouring DH should be half void entity, therefore their metamorphosis should reflect that and not a demon.
Of course all of this will depend on the specific process by which you obtain the power of the void. If they first transform into normal DH and then absorb the void like the raid boss did it would make sense, in the end it depends on the explanation they give.
Still, it feels like a lazy job that the new spec’s meta is purple Havoc.
I agree with your opinion and just to clarify, there is no anger in my comments, only criticism.