What was the role of the knights in Murder in the Cathedral?

Murder in the Cathedral tells the story of the events that led to the demise of Thomas Becket, Archbishop of Canterbury. The Archbishop is murdered in the cathedral by four knights in the last part of the verse drama. The four knights were a carrying a plot to assassinate the Archbishop, which was supposedly sanctioned by King Henry II. Thomas Becket contradicted the king’s position over the rights and privileges of the church.
The king had successfully managed to limit the clergy’s independence and reduce Rome’s influence on the realm. Although most of the members of the clergy had consented to the king’s plans, Thomas Becket resisted. Henry’s attempts at convincing Becket failed, and he instituted contempt proceeding against him. Becket lost the case and was forced to flee to France. After negotiations, he returned; however, the conflict persisted, leading up to his assassination by the four knights believed to be at the behest of the King.
The knights justify their actions at the end of the drama. They believe that they were performing their social and moral duty for the good of the public.
https://www.britannica.com/biography/Saint-Thomas-Becket

The four knights in Murder in the Catherdral act as agents of King Henry II and actually carry out the murder of Thomas Becket, Archbishop of Canterbury. At the end of the play, three of the four offer justifications for the murder. The second knight asks that they be pitied, saying they were only following orders for the good of the people of England. The third knight says that the archbishop was a guilty criminal, and so his murder was justified. The fourth knight argues that Becket brought the murder on himself, so in reality, the murder was a suicide.
The play was commissioned by George Bell, the bishop of Chichester, as a protest against the Night of the Long Knives of June 1934, in which Hitler turned on his dear old friend Röhm and had him and others who stood in his way murdered. The knights in the play represent the strongmen in Germany who carried out Hitler's orders and the justifications they used to rationalize their crime.