They are seeking 'retribution for' the killings.
The killer acted without fear of 'retribution'.
(..) I promise you, we are already well prepared for any incursions into our kingdom. I'm not sure you realize the scale of their possible retribution. Ragnar Lothbrok was not only their King, Father, he was the most famous man in their world. Believe me, daughter, (..)2017 TvShow: Vikings Title: The Great Army Season: 4 Episode 17