Yea, obviously the answer isn't refusing to fight. It's fighting a cause worth fighting for. Taking down a certain corrupt king, a king Thorfinn himself helped to become a king seems like a good cause to me.
OH NO NOT THE MANGA'S MORALE MESSAGE THAT WAS IMPLIED SINCE THE VERY BEGINNING!!! YOU'VE GOT TO BE KIDDING ME NOOOOOOOOOOOOOOOOOOOOOOOOOO. HOW I COULD I HAVE NOT SEEN THIS COMING AND CONTINUED TO READ CONSTANTLY EXPECTING THORFINN TO COMMIT VIOLENCE AGAINST OTHERS NOOOOOOOOOOOOOOOOOO!!!!!!!!