Review the Lord’s speech (3.6.24-39). How does this contribute to the theme of regicide and echo Shakespeare’s revisionist history of Scottish kings?

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.

This covers how the speech contributes to the theme of regicide. As for Shakespeare's revisionist history, Shakespeare took liberties with the historical facts of the kings of Scotland. Shakespeare wrote the play six hundred years after the actual historical figures lived. His primary source for historical events was Holinshed's Chronicles of England, Scotland, and Ireland (1587).
Some of the liberties Shakespeare takes are that the real Macbeth was on the throne for 17 years rather than the one-year timeframe in which the play takes place.
In the play, Duncan is known as wise and strong, and in reality, he was a young, weak, and ineffective ruler. In reality, Duncan became king after Malcolm's death which occurred in 1034. Malcolm did not die of regicide, but Duncan was overthrown this way in a battle led by Macbeth at Elgin in 1040. He had formed an alliance with the Earl of Orkney to wage war against Duncan.

The actual Macbeth was respected for his strength and wisdom, unlike Shakespeare's character. In 1054, Duncan's son Malcolm, who had indeed fled to Northumbria (but had never relinquished his claim on the throne) formed an alliance with Earl Siward and attacked Macbeth. He was not defeated in this battle but did return Malcolm's lands to him. Macbeth was defeated three years later in battle at Lumphanan in Aberdeenshire.
https://academic.oup.com/bjc/article-abstract/51/3/556/335045?redirectedFrom=fulltext