Regicide is the killing of crowned, sovereign monarch by another person. It includes the murder, assassination, conspiracy to murder, or execution of the monarch. It generally does not include monarchs killed in battle, such as Richard I and Richard III of England, as the monarch is playing a combatant role and is thus a fair target for enemy soldiers.
Queen Elizabeth's decision to have Mary of Scotland executed in 1587 shocked Europe in an era when official regicide was nearly unthinkable. Paradoxically, this act may have saved Elizabeth's life. When Spanish forces conqueredEngland in 1588, King Philip II refused to have Elizabeth killed and kept her prisoner in the Tower of London instead, to show that he would never stoop to such an act.