Answer:
Macbeth ordered the murder of the Macduff family, which resulted in the death of innocent wife and children.
This act is different from all the other murders of Macbeth because he has started to kill blindly, without any sense of right or wrong and thereby led to the piling up of his sins even upon the blood of innocent people.
He committed the terrible crime of killing these innocent souls just to make sure that he is safe from any opposition in keeping his authority, just as the witches had warned him.
Step-by-step explanation:
In William Shakespeare's "Macbeth", the main character Macbeth commits evil acts, in each and every act of the play. Moreover, his actions become more and more cruel as the scene progresses.
Act I shows him encountering the witches and learning of the prophecy. Act II begins the real physical commitment of evil which progresses to Act III where he continues to murder Banquo. Then in Act IV, he did the most gruesome of murder, killing the family of Macduff just to secure his line of authority. This spillage of innocent blood, who have no role whatsoever in his life or didn't pose a threat in his way to authority, Macbeth becomes the pure evil, a monster that is the result of his greed for power. Lady Macduff's protest of being innocent proves the level of evil that Macbeth is ready to take to keep his power safe.
This murdering of the Macduff family happened after the witches gave him a warning, "beware Macduff; Beware the thane of Fife". So, he took the advice of the witches literally, just to "make assurance double sure", killing not just the person they had warned him against, but also annihilating the whole family 'just in case'.
Considering the previous crimes that he had committed, they were all part and parcel of his rise to power. King Duncan, Banquo are all the obstacles that he needed to get rid of if he wants to be king of Denmark. And in doing do, he had successfully rid himself of any form or opposition. But with the killing of the Macduff family, he shed innocent blood, which is a far more greater sin than anything.