I was completely pleased with the season finale. Floki was awesome. Thinking back on this season.. everything Siggy told Horik about the value of knowing your friend's weaknesses, her desire to regain her position as an Earl's wife, etc.. she played him from the beginning. She practically told him that she was gathering intel on him.. and then she says to Rollo that everything she did, she did for him. By ensuring that Ragnar would eventually become king, Rollo would be in a position to become Earl, thus securing her regained status. She was never on Horik's side. Or if she was playing both sides, she knew who was the correct choice. The things she did to get Horik's trust were extreme, but his biggest mistake was telling her to kill Ragnar's kids and his knife was all the physical proof Ragnar needed to justify killing him. The entire setup for the final confrontation was great. Horik's wife was preparing to join a slaughter only to get destroyed by Lagertha. But Ragnar's one guy told her to leave so she wouldn't have to deal with the daughter issue. Ragnar was probably never even drunk, just like his one guy never died. Horik was completely surprised and got what he deserved. Floki.. most of the episode I thought he was about to kill Bjorn, but he was completely watching over him. When he told Horik on the beach "I decided who I want to kill".. it was Horik himself and he never saw it coming. And in the end, Ragnar went ballistic on Horik's corpse. Brutal.
I read an interview about the guy writing season 3 of Vikings: