Although the plays are written about individual kings, I think that Shakespeare used the plays as an opportunity to voice his opinion on kings and kingship in general. This was assisted by the fact that he was not prohibited by the true events, because it is well known that all of Shakespeare's plays were written purely for entertainment value, not as a historical record of what occurred.
The main notion of kingship that Shakespeare attacks in Macbeth, King Richard II and King Henry IV, is the divine right of kings, where the kings claimed that they were God's counterpart on Earth, and a 'step up' in divinity from the other aristocrats and the common people. In his plays, Shakespeare depicts the kings, and Hal, in King Henry IV, as people who were not, or at least did not act like the direct descendants of God. In Macbeth, he commits treason and murder, the two worst crimes of the day, and neither Duncan nor Macbeth were saved by God, who, according to the theory of the divine right of kings, should have saved Duncan and then Macbeth. When they were killed, they were both king, and therefore the right-hand man of God, the creator, who controls the entire world and who could have stopped them from being killed.
In Richard II, Richard bankrupts the country with his blatant mismanagement and his excessive spending on his 'favourites', who are already rich aristocrats, while ignoring the common people who are living in poverty. Richard's behaviour leads to both the aristocracy and the common people disliking him. The aristocrats disliked him because he was bankrupting the country, which they did not like because they were proud to be English and wanted their country to dominate for many more years. The common people disliked Richard because they were living in poverty while Richard was spending huge amounts of money on people who were already wealthy. What Richard did is not the sort of behaviour that is expected from God's representative on Earth, and Shakespeare makes this point quite clear to the reader.
In King Henry IV, Prince Hal associates heavily and almost solely with the common thieves and drunks in the pubs and brothels of the city. This means that he is liked by the common people, but his behaviour, particularly the robbery which he is involved in, in Act 1, scene 1, is not what is expected from son of God's representative. According to the theory of the divine right of kings, Hal should have been almost as divine and 'perfect' as the king himself, but Shakespeare shows us that this was not the case and again questions the notion of the divine right of kings through Hal's actions.
"[I] See riot and disorder stain the brow
Of my young Harry"
Through his plays, Shakespeare also shows that he is disgruntled by the way in which the kings treat the common people, by writing the plays so that the kings always appear selfish and as if they disregard the people. In Macbeth, Shakespeare shows the comparison between Duncan, a king who treated people well and was respected for it, against Macbeth, a 'fallen hero', who after hearing the prophesies of the witches succumbed to his driving ambition. Duncan listened to the people and was widely respected, whereas Macbeth ignored the people, and associated only with his noble friends. Macbeth thought that he was assured the kingship and so he thought that there was no need to listen or respect the common people in the least. It was because of the witches' prophecies that Macbeth became king, and it was the prophesies which brought his downfall - Macbeth felt assured that he would be safe,...