Is a two-month fasting expiation incumbent upon a border guard if he kills a smuggler during an exchange of fire?

There is no liability on someone who kills an aggressor who intended to assault him. Therefore, no expiation (kaffarah) or blood money (diyah) is due from him, because he killed him to repel his evil and in self-defense. If a person is subjected to an assault on himself or his property, he is permitted to defend himself by every possible means. If he cannot succeed except through fighting and killing, he is permitted to do so. Scholars have issued fatwas that whoever is killed while defending his property or honor is a martyr.