One thing that always bugged me is that the Ascians are named after the Scions of LIGHT of FF12's Espers...
( http://finalfantasy.wikia.com/wiki/E...al_Fantasy_XII) )

I think the Ascians we know were previously Warriors/Scions of Light just like we, the playing characters, are now.

Maybe having the Echo and/or Hydalyn's blessing somehow "drains" the light of those who use it, and eventually they (we) become beings of pure darkness, like the Ascians we have fought.

The only way to become a "normal" being again would be having the Anti-Echo/Blessing of Darkness, perhaps from Zodiark, so that it "drains" the darkness of them/us...

I believe that's pretty much what happened to Derplander and his buddies. They are on the way of becoming pure darkness Ascians, but to avoid that, they purposefully spend their "Blessing of Darkness" to kill primals so the darkness inside them can be "drained".