Duncan addresses the crowd with "sons, kinsmen, thanes" (1.4), and in his final speech Malcolm repeats the greeting by saying "my thanes and kinsmen" (5.8), the echoing of this address shows that Malcolm holds the same level of respect and care for his people as his father did. He welcomes everyone into his speech--as Duncan did earlier--and makes it clear that he appreciates and respects each of them by increasing their titles. Duncan in the beginning similarly showed his appreciation to Macbeth, Banquo, and Malcolm--by giving more titles to Macbeth, jewels to Banquo, and the title of "Prince of Cumberland" to Malcolm. The way that they handle the traitor (the Thane of Cawdor first and then Macbeth at the end) also is similar. They make it clear that they had trusted those men, but that those who fell into their evil or ran from them will not be punished. This shows more of how caring and kind they are in their position as king.
Malcolm's speech unifies the play in a couple of ways. Thematically we get to see that theme of power, ambition, and fate vs free will come full circle. Malcolm was named next for the throne, and then Macbeth derailed that through the course of the play by trying to take his fate into his own hands. Malcolm's speech as he becomes king shows that his position was inevitable and that power and ambition can only get a character so far before he falls. King was always going to be Malcolm's fate, it just took longer for that to happen.
In his speech in act v, scene 8, Malcolm is portrayed as a just man. He acknowledges the good actions of his men, and he ensures they receive the reward they deserve. Duncan did the same at the beginning of the play, by rewarding Macbeth and Banquo with their new titles. At the end of the play, in Malcolm's words, order is re-established, and the crown is restored to its owner.
