Murder is the unlawful killing of a human being with "malice aforethought."  Cases involving multiple murders may be known as mass murder, serial murder (or, more informally, killing sprees), or genocide. In the most Western legal systems, unlawful killings not involving "malice" are termed manslaughter.
Murder in the Bible
Believers tend not to ascribe the term murder to the many killings committed by God in the Bible.