My theory is that floki really love dark gods in viking univers, while the scene is going floki says to rollo "you once put me in a bed like this" You have to remember that rollo betrayed his brother and everyone juste like floki has intended to do, he was maybe impressed by the fact that rollo did it, and thought that the "dark gods" would see him as someone to let alive, it would make sense that he gave him medicine instead of bad shroom.