Originally Posted by
Qualia
There are a lot more "pure" demons (I assume you meant the one who were born demon in the Nether) than just the Nathrezim - Void Hounds, Abyssal, Infernals, Pitlords, etc. Also, being infused with Fel doesn't make you demons, otherwise a lot of the orcs would have turned into demons. You need to cross a certain threshold that seems to be varied in a case-by-case basis.
No way Mannoroth is more powerful than Illidan at his peak in "Illidan", much less Deathwing. Gul'dan - even now - certainly isn't as powerful as Deathwing either.