Just because he chose Ivar's side once doesn't mean he's stuck with him forever. Maybe Hvitserk should have gone with him. No one is shocked that Harald has his sights set on overthrowing Ivar, right? I chose your side, and now, for good or ill. I chose your side, and now, for good or ill, you're stuck with me. Ivar: Well, just a thought, why don't you go back to England with King Harald? What do you think they would do if you even touched a hair on my head? The people of Kattegat don't love you, Ivar. Hvitserk: Just remember, I am also a son of Ragnar Lothbrok. King Harald is coming for that very throne, unbeknownst to Ivar and the rest of Kattegat, though pretty obvious to the rest of us. If that is indeed her goal, she has some competition. If so, perhaps she plans to take out the sons of Ragnar and claim the throne of Kattegat for herself. I have to assume it was her idea to sacrifice Hvitserk.
Related: Vikings Season 5 Episode 12 Review: Murder Most Foul Does she just want to be Ivar's queen and orchestrate everything to gain his trust and be his bride? Or, does she have bigger ambitions in mind? I'm not even exactly sure of Freydis' end game.