If I got to choose I would consider it a murder, but, by definition of murder is "the unlawful premeditated killing of one human being by another". Which it does not go against the law there. Which in my eyes just seem ridiculous. It's still people killing each other, why should the location of it, or the objective make a difference, but that's just my thoughts.
Hosea Biglow, the 19th century farmer in poet James Russell Lowell's book "The Biglow Papers," says that war and murder are one and the same. He feels the justifications for war are flimsy and unsubstantiated. Biglow sadly and cynically declares the true purpose of war to be based in greed and covetousness.
Perhaps Lowell's character had a point, but going by strict definitions, Biglow was wrong. Murder isn't simply the act of killing someone. It's a legal term that defines an unlawful killing. The adjective unlawful makes all the difference. One person shooting another on the streets may be murder, but on the battlefield it's condoned.
In a war in which the involved parties have all openly declared hostilities, killing enemy soldiers on the battlefield is legal. Since murder is illegal by definition, killing a soldier on the battlefield in a war can't be murder. A soldier who kills an enemy under the rules of war isn't a murderer.
It's important to remember the rules of war. There are conditions under which a soldier could be charged with murder during wartime. The Geneva Conventions are a famous set of rules that govern what is and isn't legal during war. The rules of war dictate that it's illegal to fire upon a surrendering, unarmed or otherwise helpless enemy, or to attack any building used as a hospital.
It may seem counterintuitive that war should have rules that everyone follows. Is there really a civilized way for us to kill each other? Why is it legal to end a person's life under one set of circumstances in war but illegal in a different scenario? If the reason for the war hasn't changed, and the two or more nations -- or parties within a nation -- are still in conflict, why is one form of killing allowed when others aren't? Is killing an armed enemy soldier on the battlefield somehow less wrong than using a white flag as a pretense for an ambush?
Philosophically, you could argue that murder and the act of killing during war are the same. In both cases, the life of one person ends at the hands of another. The victim isn't choosing to die. But because war is a state in which countries sanction the use of armies of human beings in armed conflict with one another, it doesn't fit the legal definition of murder.
Why would you kill one of your friends? It is not considered murder by the courts if it is in self defense. These soldiers are just trying to defend themselves and their country. It is the same principle as self defense. Killing is part of war, so it is completely justified.
This question isn't very specific on the circumstance. Soldiers going to war against another country's soldiers? Then no, it is not murder. Murder is unlawful. The Government sends the soldiers to fight, permitting them to kill, therefore it is not unlawful. If it is a soldier killing another solider from the same country, then yes, it is unlawful, and therefore murder.