What Really Happened (spoiler)

Here's what I think really happened:

Adam and his Dad were right, they were being told by God to kill the demons and the reason Fenton doesn't see or believe it is because he is a demon, like God told his dad.

Fenton was to become a serial killer and God knew this and probably told his Dad and Adam, but as Adam says at the end, he knew for years but he wasn't on his list so to kill him before was murder.

Fenton knew as well, maybe that's what God told him in the cellar or maybe it was the devil that spoke to him, or maybe he went crazy and realized, that's why Fenton says to Adam if you need to DESTROY me bury me in the rose garden, Fenton knew he was a demon and he knew Adam was a slayer.