The murder of Macduff's family in Act 4.2 of Shakespeare's Macbeth reveals new heights of Macbeth's ambition, and of what he is politically willing to do to maintain power.
Perhaps I should say "new lows" of ambition and political intrigue are revealed. The killing of Macduff's family serves no strategic purpose. It is petty and motivated only by Macbeth's being insulted by Macduff. It is revenge ordered for petty reasons, and is not directly related to Macduff's going to England to join Malcolm. Macbeth is, in effect, being a big baby here, as well as a bully. He can't get to Macduff because he is in England (a more powerful nation), so he lashes out at Macduff's family. Of course, such immaturity and ruthlessness and impetuosity are dangerous in a tyrant.
Of course, dramatically, this kicks up Macbeth's level of evilness. Ordering the deaths of Banquo and Fleance is bad enough, but at least they were, indeed, in the way of Macbeth's maintaining power for himself and any heirs he might conceive. But ordering the death of Macduff's family is without reason and petty. And it displays Macbeth's evil nature to be worse that even previously revealed.