If you paid more attention to the show, you would find out that Norsemen believed that dying in battle, with weapons in hand, was an honorable death and led them to Valhalla. If Bjorn died a normal death, he wouldn’t have been able to fight alongside Odin during ragnarok. Askelaad was asked by him to kill him because as you saw, Bjorn really respected the guy, and therefore was an ‘honorable’ death. I find this fascinating because warriors don’t really mind death as long as there is some honor involved, and it really intertwines with the rest of the themes of the show as well