Wow. That does make a whole lot of sense when you put it like that. Original Illyria was cannonically a cthonic-type being, being put together with Fred's soul would have a huge difference in their outlook.

There's a brilliant fic series that explores these themes that you should read at Twisting The Hellmouth called the Compelled series.

http://www.tthfanfic.org/Series-461