In Act IV we see how far Macbeth has descended when he orchestrates the killing of Macduff's family. These people are not a threat to Macbeth, but he has them killed only because of their relationship to Macduff. When Macduff hears of the fate of his family, he vows to kill Macbeth "But, gentle heavens... bring thou this fiend of Scotland and myself, within my sword's length set him. IV,3
It is ironic that the killing of his family is what fuels Macduff's rage in Act V which results in the death and beheading of Macbeth.