I'm assuming this is a bug.

Early in a game a male offspring of mine had married a female child of Magnar (sp?). Magnar was eventually destroyed by a third faction. Some time later a child was born to my faction from that coupling and that child, upon coming of age, decided to align with the now destroyed Magnar faction instead of me.

This NPC was initially located at my start city (not in it though) where all my other children come of age. He was neutral but when I examined him it said he was of Magnar in his player card. I was able to talk to him and when I did the Magnar diplomacy screen came up with the normal diplomacy actions available however there was no "person" depicted on the diplomacy screen.

I wanted to get rid of him so I declared war an that faction and was able to attack and kill him. Once dead though it did not say the faction was destroyed and I remained at war with Magnar.