Regicide is the deliberate killing of a monarch. It may be difficult to imagine the impact of that living in 2018 and in a constitutional republic, but in the middle ages, regicide was a very real danger for ruling monarchs. Cambridge University Professor Manuel Eisner conducted a study about regicide in Europe entitled "Killing Kings: Patterns of Regicide in Europe, AD 600-1800." This work is a statistical study of 1513 monarchs in 45 countries. He outlined four different scenarios for regicide and noted that 15% of all monarchs fell victim to one of these scenarios. They were: murder as a means of succession, murder by a neighboring ruler, personal grievance or revenge, and murder by an outsider.

In Shakespeare's Macbeth, regicide is plotted and carried out by the title character for the purpose of succession—with Duncan gone, Macbeth will rule Scotland.

In the Lord's speech in act 3, scene 6, regicide is once again being plotted. This time, it is MacDuff who is plotting to overthrow Macbeth. Malcolm, Duncan's son, is the rightful heir and is living in England. MacDuff appeals to English King Edward to help him form an alliance against Macbeth with the people of Northumberland, Malcolm, King Edward, and Lord Siward. He says it is for the following purpose:

That by the help of these—with Him above

To ratify the work—we may again

Give to our tables meat, sleep to our nights,

Free from our feasts and banquets bloody knives,

Do faithful homage and receive free honors.

All which we pine for now. And this report

Hath so exasperated the king that he

Prepares for some attempt of war.