Vikings' King Horik is deposed and decaying but his son lives on, a choice that Ragnar may grow to regret.

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As filming begins for season 4 and Ragnar (Travis Fimmel) prepares to take on the heavy burden of responsibility that comes with his new position as King, series creator Michael Hirst took a moment to explain why the warrior Viking decided to act so leniently towards a boy who could lay claim to the throne.

"[Ragnar or Horik] had to kill the other [in the finale], and again it's made clear that they have to kill everyone," Hirst explained. "You have to kill all the family, because if you leave one alive, that person ultimately will come back for revenge. And yet, Ragnar spares Horick's son and the main reason he spares him is because he's seen the son with some Wessex earth in his hand and so he feels the son is more like him than he is like his father."

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Although the young prince may be of a similar mind when it comes to Viking expansion, that doesn't mean he won't cause problems in the future.

"It's part of Ragnar's great intelligence that he worked out a way of diffusing and defeating Horik, but a lot of the issues that came up and the way that he dealt with his son and his wife...it's one of those things that as a writer you want to do," Hirst explained to Entertainment Weekly. "You want to resolve it at one level, but you don't want to resolve it at every level. So there's plenty of meat on the bone, and there's plenty to start discussing in the new season."

Find out more when Vikings returns to the history channel in 2015.