Macbeth is a very egocentric, power-hungry leader. He starts off by being a loyal follower to his king, hesitant to act against Duncan. As Lady Macbeth says in Act I, he has the ambition but lacks the drive to do what is necessary. However, after he becomes king, this attitude changes greatly. he becomes like a dictator. Macduff seeks out Malcolm because of how poorly Macbeth rules. Had Macbeth been a good, able leader, the revolt against him would not have been so strong or fierce. Macbeth becomes very proud, largely because of his false interpretation of the witches prophecies. he degrades those who express fear or concern, and he waves off important information given to him. His whole concern is keeping his power at all costs. This extreme grasping at power that should not belong to him is part of what leads to his downfall.
First, Macbeth's willingness to kill innocents paints him as a tyrant, wielding almost absolute power and capable of immense cruelty. At least he has a reason to kill Banquo and Fleance—it isn't totally motivated by malice—but he has no reason to order the murders of Macduff's wife and children except for pure, unadulterated viciousness.
Second, Macduff's description of Scotland under Macbeth's rule presents him as a ruler that has driven his country into the dust with his own selfishness and greed. Macduff says, "Each new morn / New widows howl, new orphans cry, new sorrows / Strike heaven on the face, that it resounds / As if it felt with Scotland and yelled out / Like syllable of dolor" (4.3.4-8). Scotland is, apparently, in such bad shape that men can be killed out of nowhere (presumably, by Macbeth's orders or because men must rob one another to support their families and themselves—a situation indicated by Macbeth's earlier conversation with the men he's hired to murder Banquo), that heaven itself feels so terribly for the country that it cries with it.
Further, Macduff says, "Bleed, bleed, poor country! / Great tyranny, lay thou thy basis sure, / For goodness dare not check thee" (4.3.32-34). He names Macbeth as a tyrant and insists that Scotland will continue to suffer under his rule because even good men are afraid to stand up to him. It is ironic that Macbeth is basically ruining the very country he wanted so badly to rule; perhaps it was really only the king's position that he craved and not the responsibilities that go with it. In any case, he is presented as the worst kind of ruler who puts himself ahead of his country and his people's needs.