I've been thinking of this for a while now.
In Supernatural, it's stated that demons were once human, a long long time ago. They were killed humans who, after spending ages in Hell, have transformed into demons. But when you "kill" a demon, someone whose already dead and has become something far from human ... where the heck do they go? Ever since Ruby's Dagger and the Colt became more and more used, I'm just wondering where's the logic in that if there is any. Do they repeat the whole "get punished and become a demon" cycle?
I'm assuming it's the roughly the same cycle with the angels.