Final answer:
Macduff can kill Macbeth because he was born by caesarean section, circumventing the witches' prophecy that no man of woman born could harm Macbeth.
Step-by-step explanation:
Macduff is able to kill Macbeth due to a prophecy revealed to Macbeth by the witches, which stated that no man born from a woman would be able to harm Macbeth. However, in a twist of fate, Macduff reveals that he was "from his mother's womb untimely ripp'd," meaning he was born by caesarean section and not 'of woman born' in the customary sense. This technicality allows him to evade the witches' prophecy and ultimately defeat Macbeth, fulfilling the expectations of justice and retribution for Macbeth's tyrannical rule and the murder of Duncan, among other heinous acts.