It's pretty tough for a berserker like him to die in bed of old age. Usually, you'd expect those types to become mercenaries and join the losing side in a war so that they could go out in a blaze of glory. Except, I think he already tried that.
I have a feeling that Thorkell will soon become a villain. He is the polar opposite of Thorfinn's vision of peace and pacifism. Thorfinn may be made to kill once again, because I just do not see Thorkell ever becoming a "true warrior."