A person who commits murder is called a murderer. The legal definition of "murder" and "manslaughter" may be different in different countries, and is very much argued on: for example, killing in war is not usually called "murder" by those who fight in the war. Killing in self defense (if people being attacked with death seeming imminent kill the attacker, or someone kills an intruder who has burglarized their dwelling) is usually not "murder", but sometimes manslaughter. Many 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.