rēgicīdium

Latin rēgicīdium combines rēx (king) and -cīdium (killing, from caedere, to cut or to kill). The word names both the act and the person who commits it: a regicide kills a king; the killing itself is also a regicide. English adopted the term in the 1540s, but it became a household word a century later.

On January 30, 1649, Charles I was beheaded outside the Banqueting House in Whitehall. It was the first legal execution of a reigning English monarch — a king tried and condemned by his own subjects. The fifty-nine men who signed the death warrant were immediately called regicides. After the Restoration in 1660, the surviving regicides were hunted down. Some were executed; others fled to New England and lived under assumed names. The bodies of Cromwell, Ireton, and Bradshaw were exhumed and symbolically hanged.

The French Revolution repeated the pattern. Louis XVI was guillotined on January 21, 1793, in the Place de la Révolution. The Convention voted 361 to 360 for execution — a margin of one. The vote made every member of the Convention a potential regicide and ensured that no one could safely return to the old order. The killing of the king was a point of no return.

Regicide has been committed in every century and on every continent. Tsar Nicholas II was shot in a cellar in Yekaterinburg in 1918. King Faisal of Saudi Arabia was assassinated by his nephew in 1975. The word persists because the act is rare enough to demand its own category and consequential enough to reshape nations.