OK, double-post, but a thought just struck me based on another thread.
Murderghost (currently in it's evolved form of Sleipnir) is leading an army of ghosts and Beasts towards Odense.
We know that at least two members of the crew aren't immune to the Illness, one of them is Tuuri. We know that Onni has a very fierce love for his sister. He begged her not to leave Keuruu. It was enough to get him to leave Keeuru and travel all the way to Mora.
So, Tuuri is killed (or worse, comes down with the Illness).
Onni is emotionally devastated by this. He blames Lalli, of course, since Lalli should have protected her, but he mainly blames himself for letting her go.
Onni becomes a kade out of grief and anger.
Horrible thought, I know