Shakespeare was a man of his time and his plays required the support of the Establishment in London. As a result the historical stories were often edited to the current politically correct.
In Scottish history, MacBeth is listed as one of the good kings. Where the Bard got it wrong is that MacBeth was the rightful king and Duncan the usurper since kingship in "Pictavia" was matrilineal, and did not become patrilineal until after the first union of Picts and Scots produced a king who inherited kingship from both parents. If you read between the lines, the reason there are a lot of evil uncles in Scottish history is because so many matrilinial descents went to minors with jealous uncles as regents.
Aside: several years ago I taught a high school summer class for kids who had failed English at some level. Because they were all in the same class, I used an old text for lit that contained no selections currently used in the four grades, MacBeth being one of them. A week later, in Scotland, heading toward St Andrews, I was crossing the Forth road bridge when I got an idea, got off at the Kirkcaldy/Dunfermline roundabout, drove down to Kinghorn, hired a boat, and headed for the base of the northernmost of the two towers supporting the Forth bridge. Standing at the base of the tower at the west end of the island, I took a photo of the ruins of Inchcolm Abbey at the eastern extremity of the island, scene of Act I, scene 1.
As I was to be in Scotland for the rest of the year, on my free time, I went to all of the sites mentioned in the play and photographed them. Shakespeare must not have had a very good map. He also played fast with the Scottish language to fit his rhyme and meter. While all the locals knew of "Birnam Wood" none had ever heard of |dun-sin-AIN|, until I discovere that, pardon Shakespeare, it is pronounced, |d´n-SI-nun|.
If you ever go to Glamis, don´t let the guide get away with any connection between MacBeth and the Castle, On the drive in, what is left of MacBeth´s castle is just some rocks on a hill to to your right about 2 miles from Glamis Castle. Like many Scottish Lairds, who did not like the peasants too close, the present village of Glamis was rebuilt by the landlord about a mile from the Castle, thus making three Glamiseseses.