For the entirety of part I Thorfinn is obsessed with with quest to avenge his father. Having witnessed Askeladd's death at the hands of another, he becomes a shell of himself, ultimately enslaved and passive. To me this is a great metaphor for obsession; define yourself by one thing and there will be nothing left of you when it's gone. Even if Thorffin had killed Askeladd himself, what would have been left of him? Would he have remained a henchman, become an assassin? Having focused his entire life on one purpose was his personality and sense of self well enough developed to have any sense of purpose or drive after completing his revenge?