Macbeth sends murderers to capture Macduff's castle and to kill Macduff's wife and children because he sees Macduff as a threat to his power and wants to eliminate him. Macduff is one of the most prominent figures opposing Macbeth's rule, and he is seen as a potential leader for the opposition forces. Macbeth hears about Macduff's flight to England to join forces with Malcolm, the son of the previous king and rightful heir to the throne of Scotland. Macbeth believes that by killing Macduff's family, he can punish him for his opposition to his rule and weaken his resolve to overthrow him.
Macbeth's decision to send murderers to kill Macduff's wife and children is a turning point in the play, as it marks the end of any sympathy the audience may have had for him. It is a brutal and senseless act that illustrates the extent to which Macbeth has been corrupted by power, and how he has deviated from the honorable path he once followed. Ultimately, Macbeth's actions are his undoing, as they set in motion a chain of events that leads to his downfall.