uhm, I don't totally agree. From your point of view Seiya or Saori should have killed this baby merde :
Yes, Koga still looks human in this form, while Kurumada draw an... Alien?!?!?
But if he would have represented a supernatural entity / only a spirit, Kurumada would have drawn a "galaxy" thing, like Hades.
Instead the third twin as an humanoid appearance.
And if you read the dialogues of this chapter, Dokho never talks about an evil spirit (it's Ker informing us about)
Dokho talks about a star, that takes possession of a good star, and ultimately Dokho shout Shion to kill
the baby.
Imho this case is pretty similar to Koga's one or, if you prefer, an evil star of a specter arising in a toddler, instead of an adult.