Okay, so I watched the most recent episode ("The Choice") on Xfinity OnDemand because I missed the episode Thursday night and I know that many of the episodes on OnDemand include a sneak peek for the seguinte episode at the end.
Well the clip for seguinte week's episode was rather nerve-wrecking.
In the sneak peek, it started with Siggy watching over a bedridden Rollo and Floki has come to supposedly visit. Floki convinces Siggy to go rest because she deserves it and she finally leaves, trusting Floki to watch over Rollo.
Floki sits at the side of the bed, looking at Rollo (who wakes up), and Floki talks to him a bit before shoving what looks like mushrooms into Rollo's mouth and giving him some water to wash it down. It then shows Rollo starting to cough or hack, and the clip ends.
So it appears that Floki may be poisoning Rollo in the seguinte episode.
Now, I know that Floki never really forgave Rollo for what he did, but he knows that Rollo is Ragnar's brother and that Ragnar loves him. If the clip is true, and Floki does poison Rollo and kills him, this would mean Floki's betrayal is real.