Macbeth betrayed Scotland by lusting after power for power's sake, rather than wanting to use his power and position to benefit and support others, which should have been his role as king. From his act of regicide when he killed Duncan to his other acts of murder and crime that he committed in order to maintain and support his fragile hold on power, Macbeth only sought to serve his own interests and needs, and never thought of using his power to benefit others. This is the major way in which Scotland is betrayed: ultimately it does nobody any good to have a leader whose only aim and purpose is to maintain power for himself. Note how Lennox talks about the need that there is for an outside force to invade Scotland and redeem it from the terrible rule of Macbeth in Act III scene 6:
Some holy Angel
Fly to the court of England, and unfold
His message ere he come, that a swift blessing
May soon return to this our suffering country
Under a hand accurs'd!
The language that Lennox uses in this quote is very powerful in the way that it communicates how Scotland as a country is suffering greatly because of Macbeth's rule. It is a country that is "suffering" and it is "under a hand accurs'd." Macbeth's power crazy habits have cursed the country that he would sacrifice everything to rule over. This displays the way in which Macbeth's hunger for power was very short sighted and not embarked upon for any other reason except his own: his evil reign has cast a blight on all of Scotland.
As a thane (MacB's position), he's a political leader, similar to a governor. By him betraying the king and murdering him, he's going against the better interest of his country and focusing on his own desires.