They are seeking 'retribution for' the killings.
The killer acted without fear of 'retribution'.
(..) There is much talk of the revenge of the sons of Ragnar. I know what to expect. And we are fully prepared. I'm not sure you realize the scale of their possible retribution. In the name of our dead father, in the name of Ragnar Lo'br'k, we declare war on the whole world. ~ Season - Episode ~ (..)2017 TvShow: Vikings Title: Revenge Season: 4 Episode 18