Originally Posted by
A Chozo
1 - All the big Sha have their own particular personality. They aren't exactly extensions of Y'shaarj. It's a curse made manifest, containing some shadow and bones to have mass.
2 - It's still different from how demons work. You kill an Old God, as far as we know, it remains dead, not going back to anywhere. If you kill a demon, it respawns. One is just too stubborn to die, the other can die multiple times and come back - except if you kill the demon in the Twisting Nether.
This difference is important. It means that there is a way to purge Azeroth from Old Gods without going into the realm of the Void Lords to destroy their Old God factory. We just need some sort of... planetary shield defense system to keep any further Old God alway from Azeroth, otherwise we'll still be at risk of having a Metroid Prime's Leviathan crashing onto us to corrupt Azeroth with their Old God Shit / Phazon.