Complex sentence: Major Kang, who knows he will be hunted down as soon as he returns to the present, goes back to the village to help defend it from the Jurchens; the other modern Koreans stay behind for their own reasons as well.
Split into simpler sentences: Major Kang, who knows he will be hunted down as soon as he returns to the present, goes back to the village to help defend it from the Jurchens. The other modern Koreans stay behind for their own reasons as well.