You will find it helpful in response to this question to analyse the function of the Chorus in Greek drama and compare it to how Eliot utilises the Chorus in this masterful example of modern drama. The Chorus seems to function as a kind of mediator between the action of the play and the audience. In this play, it is made up of a group of women of Canterbury. They are actors in the play itself, being involved in the plot, but also comment upon the action of the play. Thus it is that they beg Thomas to return to France, fearing his death, and they also provide a commentary on the socio-economic position that they occupy and the kind of miserable lives they lead under the rule of King and Barons.
However, fascinatingly, they develop as a character as the play progresses, and by the end of the play, they recognise the way in which there is a greater force at work than the human interventions they are witness to in the messy world of politics and they end up in the conclusion of the play poignantly affirming this by singing a hymn of praise and adoration, extolling the wisdom of God:
We thank thee for Thy mercies of blood, for Thy redemption by blood... the blood of Thy martyrs and saints shall enrich the earth, shall create holy places.
Even the cruel and apparently senseless death of Thomas and the defiling of the Cathedral can therefore be interpreted as part of a bigger plan that we are not aware of, and the appeal to the wisdom of God that the play ends with helps us to have faith in a larger plan that is beyond our imagining and conceiving, but nonetheless weaves such violent episodes into something beautiful and stunning.