The scene containing the knights' speech provides comic relief, and its function is to force the audience to think about the gravity of the situation. The knights and their justification of the murder of Becket demonstrate moral bankruptcy of the earthly power, represented by the king.
After Becket is murdered, all four knights attempt to justify the murder before the audience. The first knight is Reginald Fitz Urse. He insists that he is the man of action and not so eloquent, which is why he wants to let other knights speak. The second knight, William de Traci, gives a speech next. He describes himself and the other three knights as "four plain Englishmen who put our country first." He insists that the four of them did not get a penny for the slaying and only did what was right for the people and what was required by the king. The third knight, Hugh de Morville, speaks next. He explains thoroughly that the knights were only doing what was desired by the people, like those sitting in the audience. Becket's subordination to his own beliefs conflicted with his role as a servant of the king. He disregarded the world of orders and authority and that itself was why he had to die. And the last knight, Richard Brito, cunningly states that the knights are innocent because Becket committed a form of suicide. Becket's desire for martyrdom caused it, and he rejects any moral responsibility imposed on the knights for the committed murder.
These speeches emphasize the neglect of the spiritual world by the knights and the importance they place on the temporal world of order and power.