The origin of this quotation, which I have heard in different varieties, is fascinating, and involves Justice Holmes!
In the early 1860s, Oliver Wendell Holmes Jr., then a brash Harvard undergraduate, wrote an essay criticizing Plato, whose classifications of ideas he found “loose and unscientific.” Holmes sent a copy of the essay to Emerson, whose books, he later said, had “set me on fire.” He soon received in return a nugget of stern wisdom. “I have read your piece,” Emerson replied. “When you strike at a king you must kill him.”
I forget that Holmes was old enough to correspond with Emerson! At the time, Emerson would have been in his 60s (he died in 1882 at the age of 78).
In any event, this old maxim was proved in Bulgaria where a gunman attempted to assassinate the leader of a party, Ahmed Dogan. The would-be-assassin jumped up on the stage, drew a gun, pulled the trigger, but it jammed. It looks like a tiny subcompact from the picture.
Dogan knocked the gun out of his hand, and fled. Then security forces proceeded to kick (literally) the crap out of the gunman for several minutes. So if you are going to shoot the king, don’t jam.