Early in the story, a girl, Ragna, is attacked (presumably raped).
It seems like it must be either Sigurd (because Somerled made him think it was appropriate behaviour) or Somerled himself.
Sigurd attacks Somerled as a result, but that could be because he's angry at being tricked by him, or angry that he attacked the girl. Sigurd is also upset at her death, but that could be because he's angry that it's his fault, or just that she died.
Later, Somerled presumably kills Sigurd. I don't see why he would do that if Somerled himself was the attacker.
Somerled denied doing it when asked by Eyvind, and despite everything else, he was generally honest with Eyvind.
There's also some mention of it late in the book, but I don't remember exactly what it was (and don't have a copy handy any more). If I remember correctly, it only left me in more doubt about whether it was Somerled or not.
Is this answered in the book? Or at least made clearer than it seems to me?