The events in Kattegat were something no one could have conceived happening, Tyr alongside Erik had just been sentenced to be exiled.
Ragnar was filled with rage that his son would side with the man that violated his mother and caused her so much pain. Bjorn felt betrayed, and Lagertha was the only one with a heart big enough to understand where her son was coming from but she knew the consequences that came with his actions.
The sentence had been given and the fact that Askild allowed the citizens to decide their fate not only undermined Birger as the supposed King but took away what little chance he had left of the control he thought he had. Birger had only one course of action and that was to kill Askild, he charged toward the injured man, no one was within proximity to stop his attack, Askild had to defend himself yet he remained calm.