How does Shakespeare's Macbeth show us a true leader should not resort to the misuse of power?

Shakespeare's Macbeth shows us the beginning and end of Macbeth's downfall once he embraces evil. 

At the beginning of the play, Macbeth is a loyal and valiant warrior, praised by everyone, including the king of Scotland. He is given honors for his honesty and bravery, and our first impression is that things could not go better for Macbeth. 

Once we examine Macbeth more closely in his soliloquies, however, we begin to understand he harbors some...

Shakespeare's Macbeth shows us the beginning and end of Macbeth's downfall once he embraces evil. 

At the beginning of the play, Macbeth is a loyal and valiant warrior, praised by everyone, including the king of Scotland. He is given honors for his honesty and bravery, and our first impression is that things could not go better for Macbeth. 

Once we examine Macbeth more closely in his soliloquies, however, we begin to understand he harbors some perilous ambitions which could change his life for good. His tragic flaw is his unrestrained ambition because he is ready to cast aside all the true values for the sake of gratifying his needs. He knows that in order to become the king, he must resort to the most illegal means—he must kill Duncan. 

When Macbeth murders the king with his manipulative wife's help, his life begins to change for the worse. He manages to fulfill his ambition of taking the throne, but he loses other people's support and his own inner peace, stability, and compassion. He turns into an evil murderer, ready to have anyone assassinated if doing so will ensure his security.

Macbeth's abuse of power leads to his own downfall. Macbeth is anything but powerful; he becomes obsessed with protecting himself because he knows he took the throne illegally. His conscience begins to haunt him because of all the misdeeds he committed.  

Once he accepts evil, Macbeth voluntarily deprives himself of the opportunity to lead a meaningful life. Not only is Macbeth defeated at the end of the play, he also realizes his life has become "a tale told by an idiot, full of sound and fury, signifying nothing."