Macduff does become a divine avenging angel, saving Scotland from the tyrant Macbeth, particularly because he is identified by the witches as being able to kill Macbeth.
Macduff is very suspicious of Macbeth and feels compelled to skip the coronation and return to his home. He does not pledge his loyalty and allegiance to this king.
"Macduff's words in the next scene are considered significant by some observers who argue that Macduff is the first character to suggest his suspicion regarding Macbeth's ascension to the throne. Macduff tells Rosse that he will not be attending Macbeth's coronation but will instead be returning home to Fife. After Ross states that he will be going to the coronation, Macduff replies: "Well, may you see things well done there: adieu, / Lest our old robes dit easier than our new" (II.iv.37-8)."
Macduff's role in Macbeth turns into savior because of the fact that he is aligned with Malcolm, the true heir to the throne after Duncan. Macduff goes to England to seek out Malcolm and together they get help from the English King to invade Scotland in order to restore the throne to the rightful line of succession.
Macduff is further motivated to overthrow Macbeth because the tyrant King had his entire family murdered. When Macbeth sought out the help of the witches for the second time, they warned him about Macduff. Beware Macduff they told him, he did not know what to make of this so he sent killers to murder him.
Macduff was not at home when the murderers arrived at his home, so they killed everyone else in the house instead. When Ross brings this tragic news to Macduff in England, he is so taken back, that he pledges to kill Macbeth to seek retribution for his murdered family.
So Macduff has more than one motivator that spurs him to become the avenging angel in Macbeth. He is very courageous in his fight with Macbeth, finally beheading the tyrant king, thereby restoring order, justice and law to Scotland. Malcolm is crowned king because Macduff killed Macbeth. Only Macduff was capable of killing Macbeth, because as the witches said, he was not of woman born, meaning that his birth was by Cesarean section rather than natural birth.
"Macduff faces Macbeth filled with rage and vengeance. When he tells Macbeth that he was “untimely ripped” from his mother’s womb, Macbeth realizes that the Witches gave him information that had a double meaning. Macbeth at that moment realizes that his fate has been sealed and he is not immortal."
Macduff, marked by his extraordinary birth, becomes the hero in Macbeth.