So because undead demons exist all of them are?
That is how fel magic works. Take the dark portal of AU Draenor for example it constantly needed new souls, to operate, if souls wouldn't be destroyed they would be an unlimited power source for the portal, but the iron horde constantly fed the portal new sacrifices.1) I don't see the souls released from my Haunt spell being destroyed.
He consumed them, he did not use them.2) In the Patch 6.2 Gorefiend encounter Gorefiend releases souls he had consumed to power his demonic/fel magic, they're not destroyed.
You stated demons, all demons are undead, because of their felblood and since you can become a demon, without the blood, meaning fel would turn anyone automatically into an undead, which simply isn't true.Did I state that anything infused with fel magic automatically turns undead? No.
Meryl is possessed by a dreadlord, by the name of Kathra'Nathir.Meryl is possessed by a fel tainted elemental spirit? Nice theory.
They are a construct, like an Kirin tor familiar, but with a different purpose bound to the caster and fel based.The Eyes of Kilrogg are constructs? Constructs animated by what? Where is their physical shell/body? If they don't have a physical shell/body then they are no different from fel spirits.
http://www.wowhead.com/npc=59020/eye-of-the-legion
^The Eyes of the Legion look undead to me.