a person who commits murder is called a murderer. the legal definition of "murder" and "manslaughter" may be different between countries. a killing in war is not usually called "murder" by those who fight in the war, as it's justifiable homicide. killing in self defense is usually not "murder". some countries do not even have manslaughter as a legal concept, and only have murder; the countries that do not have manslaughter charge with murder instead. in fact, the united states did not have manslaughter on the books until the 20th century. england was the first country to indoctrinate manslaughter, and the united states was second.