Murder has two meanings. It can refer to killing which falls within a legal definition of murder at a particular time and place, which can vary widely. Or it is used to refer to other killings, with the sense that the person using it believes it to be wrongful killing.

Somtimes the two meanings can merge retrospectively - we would have no difficulty in referring to what Hitler or Saddam did as murder even if it turned out that the laws had been adjusted so that it was all nicely legal at the time.

I don't think it's legitimate to try to restrict the use of the term to situations where the law is in line with our understnading of morality.