A tale of the woes of Scotland. David [Riccio], the Queen's servant, is murdered with twelve daggers. King and Queen quarrel over this. Bothwell takes the king and hangs him. This produces such anger that the Queen flees to England
Henry Lord Darnley was Mary Stuart's cousin (and heir if she remained childless), and after their marriage he was addressed as King. Darnley is thus the "king" of this ballad and Mary Stuart the Queen. The Queen of England is, of course, Elizabeth I (reigned 1558-1603). Mary Stuart was Elizabeth's heir.
(A spelling note: The Scottish spelling of Mary's name was "Stewart." Since, however, she spent much of her youth in France, she used the French spelling "Stuart.") - RBW