Demons
Fenrir had a lot of demons hidden away in his mental closet. There were large demons: evil things that even he knew not how they got there, only that they were in him now and would forever stay there. There were small demons who weren't as bad; little demons that were made of guilt and senseless pride and sometimes remorse over killing a wolf in his pack only because it was necessary.
Greyback was a strong werewolf and that was not questioned; other wolf tribes would love to join his pack, but he only accepted the strongest and purest. If an impure joined his group, he would have lost all he had worked up for during his human and non-human life.
He understood that Remus Lupin would never make it as a true werewolf, but he turned the young man anyways; he held no remorse for it afterwards, but he knew somewhere there was a guilt-demon gnawing away at his soul.
Fenrir Greyback was a demon and he was treated as so. The demons in his mind haunted him like one and the people he allied himself with treated him like one as well.
It wasn't until he met Percy Weasley that he understood a kindness and pureness he could only dream of.
He had bit the older Weasley child during a small encounter two years before the Great Battle but had not heard of anything. So either he had died during those years or kept it top-secret, even from his parents.
Upon seeing him on the battle field, helping the light side, Fenrir understood why he had not heard a thing about the younger werewolf. Percy glowed in a strange light as he fought in his wolf body – a pure white light that almost purified the demons the wolf-spirits shadowed the human soul with.
Fenrir growled loudly, wanting to taint that light; he attacked with such vigor he forgot of the world around him and only focused on this young man.
Two hours passed and Fenrir lay bloodied on the ground as Percy stood above him, intestines sliding from his mouth and his muzzle covered in dried and fresh blood. The ooze stunk of corpse and Fenrir understood that it was his blood and intestines Percy held; coveted like a new possession.
As Fenrir lay dying, he understood why Percy was protected with that white light and Fenrir shuddered at the "could have been"'s.
He watched as his life slowly slipped from his dirty and torn body as Percy slowly made his way over to Remus, who was protected in the same white light.
Percy had never let the demons of hatred and death take over his soul like Fenrir had. Percy kept pure and mated with a pure werewolf.
Fenrir let out one last lonely howl as demons finally took away the rotted corpse of his soul.