User blog comment:Chaos11394/Where Do the Demons Go?/@comment-3481504-20120627153917/@comment-3045936-20130128185556

Agreed General. Take Angels as an example. Despite, being far more powerful than 90% of all other races. (Minus ones like the Leviathans; which are immune to the powers of some if not all orders of angels) they are still spiritual or ethereal like entities.

And as stated when Castiel said on two accounts in season 6. When Lucifer killed him before God restored him "I was done." And to Crowley. " I don't have a soul." So just being a spiritual being doesn't mean your not able to be destroy like souls. Demons are corrupted souls, but who is to say HOW corrupted are they.

Is just giving them demonic powers, making them evil spirits, it could just that simple. But at the same time it could be the corruption of being a demon shifts the soul into something else. Still spiritual but different. Also, as when Kevin read from the tablet  "The demonic influence on the collective tapestry of the soul." give hints to what demoinc corruption does in Hell. So it's possible that, the corruption of demonic influence shifts the soul from indestructible pure energy into a form of corruption and evil.