Askeladd murdered a child's father in cold blood through treachery, and then leveraged that innocent child's sadness and hatred to turn him into a tortured killing machine/human cannon ball to be shot at his enemies.. Thorfinn is raging with his desire for revenge and challenges Askeladd to a one vs. one battle where they have a very intense and gruesome battle which ends up in Thorfinn's victory, finally being able to kill Askeladd.
Essentially, Askeladd's death was not quite a sacrifice for Canute's well-being or necessarily out of loyalty (I mean maybe that was part of it but I don't think that was the point), but rather for Askeladd's hopes for the future of the Danes.. But Vinland Saga is a cruel series, and Thorfinn was never given the chance to execute his revenge. Askeladd is murdered by Canute right in front of Thorfinn, which made the boy even more furious: he had to be the one to kill him!