Sacrificing your life isn't always the moral thing to do. Take the first example; you have an agressive man (A), and a defensive man(B). If B kills A, the murder starts a war (highly unlikely, but let's assume it does and both know it). More people die until the war has run it's course.
On the other hand, if A kills B, there is a murder just like it, and a threat of war. But even if there is no war, there is still an 'agressive' person walking aound, who is bound to kill others after B, whereas if B would be the one to survive he would not voluntarily do any more killing. By defending yourself you would probably be saving lives.
In the vengeance scenario where the soldier was ordered to kill in the other country in such a way, I think talking should be attemted first. You can go around killing everyone, but that would only increase hostility, and thus attacks. If it's ineffective, close your borders to that country and file a complaint elsewhere.
Third scenario, killing people off is definately not moral. If you want to take away their ability to strike out at you, order them to disband most of their military instead. Harder than it souns, but so is 'simply' shooting everyone just because they were born in another country.
And if defending your nation isn't moral, attacking one certainy isn't either. It's often difficult to estimate what quantity of lives will be lost when you make such a decision. But even if you surrender before people get killed, most of the people in the country will not like their new government. When people are oppressed they will often fight for what they consider freedom, regardless of whether or not that takes lives. These attacks are mostly uncoordinated and usually brutal, and eventually result in more deaths than a succesful but costly defence from the start.
Or so I gathered
