This concerns episode 15: Ragnar was saying that the people split into two factions siding for one of the princes to become king. It doesn't matter who is older or even if one of them doesn't want the crown or what the king picks. To prevent infighting within his kingdom because of these factions King Sweyn decided to have one them killed. Also I think Canute is at least vaguely aware of why he was sent to the battlefield which is why he made that rant about it being "obvious that a father should love his child"