Why does Macbeth kill Macduff's wife and children?
At this point in the play, Macbeth has lost all touch with moralistic values and rational thought. He is so fearful of losing his crown that he will do anything to protect it. When Macduff was absent from Macbeth’s inauguration, Macbeth grew suspicious of him and decided to make a strong point by murdering Macduff’s family. The reason for the murder of his wife and children was to clear the bloodline. In Macbeth’s mind he did not want to lose the crown to anyone in Macduff’s family and the massacre would also send a strong message not to oppose him. There is also a possible bit of resentment toward Macduff because he was able to do one of the basic primal objectives in life, reproduce.
but still why
The play include not only the murder of Duncan, but infanticide (Act 4 sc2) and regicide (Act 2 sc2) also.We do not see the actual killing of Duncan. Shakespeare spares us that because he wants us to concentrate not upon the act itself, but on the significance of it, and the subsequent reaction of Macbeth and his wife. But this brutal killing and other even more violent actions (the stabbing of Lady Macduff and his young son), are shown to us. The effect is that we see clearly and are therefore made to 'feel' the nature of Macbeth's 'power' and the price paid by other for his ambition.
Macbeth has already embarked on a course of murder and bloodshed, as suggested by the witches. His degradation is complete. Commenting on this brutal murder scene (killing of lady macduff and his son), Coleridge writes, "the scene, dreadful as it is, is still a relief, because it is a variety, because it is domestic, and, therefore, soothing as associated with the only real pleasure of life."