The Viking world was experiencing a power shift, the attempted murder of Lagertha by Birger even though he did manage to rob Ragnar of his child.
A man grieving the fall of his other child, Tyr, in battle.
Ragnar felt the gods were against him but he was a King first and a father second.
He assumed this responsibility and couldn't bring his Kingdom to ruin due to his family being affected.
Ragnar was a foreign King so he expected resistance but not to this extent.
Ragnar knew he would never find the perpetrators if Vestfold remained at full strength.
This was why he took his men out and marched towards Kattegat, this would prevent Askild from using this chaos as an opportunity to invade his Kingdom.
Ragnar knew the kind of man he was and there was a slim chance he would pass on this opportunity.
But this also disqualified Kattegat as a potential threat.
