Regicide is the purposeful killing of a monarch or sovereign of a polity and is often associated with the usurpation of power. A regicide can also be the person responsible for the killing. The word comes from the Latin roots of regis and cida (cidium), meaning "of monarch" and "killer" respectively.
In the British tradition, it refers to the judicial execution of a king after a trial, reflecting the historical precedent of the trial and execution of Charles I of England. The concept of regicide has also been explored in media and the arts through pieces like Macbeth (Macbeth's killing of King Duncan) and The Lion King.
History
In Western Christianity, regicide was far more common prior to 1200/1300. Sverre Bagge counts 20 cases of regicide between 1200 and 1800, which means that 6% of monarchs were killed by their subjects. He counts 94 cases of regicide between 600 and 1200, which means that 21.8% of monarchs were killed by their subjects. He argues that the most likely reasons for the decline in regicide is that clear rules of succession were established, which made it hard to remove rightful heirs to the throne, and only made it so that the nearest heir (and their backers) had a motive to kill the monarch.
There is evidence that regicide and the ability of states to keep or even expand their territories are negative correlated: Firstly, elite violence hindered the development of territorial state capacity, and the killing of rulers also directly resulted in a more likely loss of territory. And secondly, state capacity, reflected by territorial state capacity, could be hypothesized to have had a restraining effect on interpersonal violence. This would be consistent with Pinker's (2011) view that modern state capacity leads to a reduction in violence, both interpersonal and in terms of military conflict.
Britain
Before the Tudor period, English kings had been murdered while imprisoned (for example Edward II and Edward V) or killed in battle by their subjects (for example Richard III), but none of these deaths are usually referred to as regicide.
Execution of Mary, Queen of Scots
The word regicide seems to have come into popular use among continental Catholics when Pope Sixtus V renewed the papal bull of excommunication against the "crowned regicide" Queen Elizabeth I, for—among other things—executing Mary, Queen of Scots, in 1587, although she had abdicated the Scottish crown some 20 years earlier. Elizabeth had originally been excommunicated by Pope Pius V, in Regnans in Excelsis, for converting England to Protestantism after the reign of Mary I of England.
Execution of Charles I of England
After the First English Civil War, King Charles I was a prisoner of the Parliamentarians. They tried to negotiate a compromise with him, but he stuck steadfastly to his view that he was King by divine right and attempted in secret to raise an army to fight against them. It became obvious to the leaders of the Parliamentarians that they could not negotiate a settlement with him and they could not trust him to refrain from raising an army against them; they reluctantly came to the conclusion that he would have to be put to death. On 13 December 1648, the House of Commons broke off negotiations with the King. Two days later, the Council of Officers of the New Model Army voted that the King be moved from the Isle of Wight, where he was prisoner, to Windsor "in order to the bringing of him speedily to justice". Then in the middle of December, the King was moved from Windsor to St James's Palace, Westminster. The House of Commons of the Rump Parliament passed a Bill setting up a High Court of Justice in order to try Charles I for high treason 'in the name of the people of England.' From a Royalist and post-restoration perspective, this Bill was not lawful, as the House of Lords refused to pass it and it predictably failed to receive the Royal Assent. However, the Parliamentary leaders and the Army pressed on with the trial anyway.
At his trial in the High Court of Justice on Saturday 20 January 1649 in Westminster Hall, Charles asked "I would know by what power I am called hither. I would know by what authority, I mean lawful". In view of the historic issues involved, both sides based themselves on surprisingly technical legal grounds. Charles did not dispute that Parliament as a whole did have some judicial powers, but he maintained that the House of Commons on its own could not try anybody, and so he refused to plead. At that time under English law, if a prisoner refused to plead, he would be treated identically to one who had pleaded guilty. This has since changed; a refusal to plead is now interpreted as a not-guilty plea.