Instead of killing Macduff as Macbeth has chosen to do to his other enemies, this time Macbeth sends murderers to attack Macduff's castle and slaughter his family and everyone else who lives there. All of the victims are innocent; many are women and children who were unable to protect themselves. Macbeth's action this time is designed to draw Macduff back from England so that Macbeth can kill him himself; because of the witches' second set of prophecies, Macbeth believes himself to be invincible.
His action this time shows that he is becoming more ruthless as king. Furthermore, killing women and children is dishonorable. His killing rampage has escalated from murdering one person (Duncan and then Banquo) to the population of an entire castle. Macbeth seems to believe he must spill more and more blood to maintain his position as king.
The killing of Macduff's entire household, his wife, children, servants is a turning point in the play for Macbeth. He is deranged with paranoia, using murder to protect his power, striking out at innocent women and children. He is anticipating an assault from Macduff, and intends to kill him and anyone else at his home in order to prevent any challenge from him.
The murder of Macduff's family begins the descent into total madness for both Macbeth and Lady Macbeth. She, in particular, cannot see the justification for the murders, and becomes emotionally isolated from her husband.
Macbeth, is tortured by fear and uncertainty and becomes obsessed with keeping his power and authority as king.