Why does Bors shouts Rus in the movie King Arthur?

Answer 1

There is a theory that Slavic people's are of Sarmatian origins, seeing as how one's dissapeared and other's suddenly appeared in history.

The Rus' warcry is probably put in the movie to back this claim, although it is very wrong.

The Rus were a Scandinavian tribe that were made leaders by Slavic tribes which constantly fought each other. By putting outsiders as the leading figurehead the tribes could unite and form a powerful state. They might have also conquered the tribes which was unlikely, due to low numbers of Rus' and vast numbers of Slavs.

This tribe is from where the Russians derive their name, but the Rus did not even left Scandinavium at the time the movie takes place.