Wow, I seem to have touched a nerve.
In my original post, perhaps I was too hotheaded about the grammar and not emphatic enough about what really bothers me: as other people have pointed out, the tone and style of the dialogue is not consistent through the films, even within the lines of individual characters. To my ear, this line of Theoden's is a modern
linguistic insertion (as well as a modern sentiment, as
Lalaith rightly points out) into a film that, while outside of any historical chronology, is definitely not set in the modern day. For me, this line is just as clunky and out of place as "Game over" or "That's because my axe is embedded in his spinal cord" or Gandalf's "on our tail" line, for exactly the same reason.
Also, I really do love these films--I wouldn't know them well enough to pick out the (relatively few) lines that bother me if I hadn't seen them multiple times, right? And Theodred's funeral is one of my favorite scenes of all the films, so perhaps that's why this one tiny linguistic nit sticks out to me as ripe for the picking.