Re: OOC
It certainly seems on the face of it that Joe is similar to the slayer. Fey do do bad things that torment and occasionally kill humans. Joe's vendetta is going to lead him to kill a load of Fey but then, unlike Christopher, who just killed a god (and no doubt a few hundred humans as collateral), Joe IS Fey himself.
Perhaps a closer comparison would be Spike after he got his soul back or maybe even better, when he decided of his own free will to try to get his soul back.