This is an interesting question, because I actually believe that Macbeth displays many kingly virtues, especially in this last Act of the play. However, in Act 5 scene 3 I think we can clearly see what his biggest downfall is: his over-confidence in the prophecies that the witches gave him leading to arrogance. Note the way that this scene begins and how it expresses Macbeth's unshakeable belief in his own safety:
Bring me no more reports; let them fly all:
Till Birnam wood remove to Dunsinane,
I cannot taint with fear. What's the boy Malcolm?
Was he not born of woman?
He places all of his confidence in the "truth" of the prophecies the witches gave him, and he concludes his opening speech with a strong ending that makes clear his resolute spirit in the face of the betrayal of some of his lords who are defecting:
The mind I sway by, and the heart I bear,
Shall never sag with doubt, nor shake with fear.
These are noble words and strike us as the audience with the bravery and nobility of Macbeth until we remember that he places his confidence in black sorcery rather than something else better suited to be the receptacle for his hopes. It is this that essentially leads to his downfall. The arrogance that these prophecies create in him shows his lack of humilty: an essential kingly virtue.