Duc and Linh have been killed. This leaves Bon with no sense of purpose. Having given up his weapon, he’s no longer a soldier. Having lost his family, he’s no longer a husband or a father. His only remaining identifiable connection is to the narrator. The circle of blood that expands on Linh’s chest comes from her being pierced in the heart. The wound is also symbolic of Bon’s heartbreak, and the bond that existed between Bon, Linh, Duc, and the narrator just moments ago, but which has now been broken.