True, self defense isn't called murder. But if the government drone strikes an American citizen without a trial or anything, that's "extrajudicial killing", not murder.
Murder is a universal concept. There are also varying criminal laws that are called murder, but just because these exist, one must not be thrown off track: the moral, pre-legal concept of an act known as murder remains unaffected.
'Extrajudicial killing' is just an apologetic euphemism. An indirect term, since murder is usually considered to be a bad thing.
