If I were to write an analytical essay on Shakespeare's Macbeth, I would find a theme that is easily supported. I would concentrate on how the promise of power changes a valiant servant of the King to a murderous tyrant. This theme is something that is timeless: we see it happen all the time—where power (or money, or both) lure a good person into a life of crime.
If you concentrate on one aspect of the play, I would analyze the man Macbeth was and who he becomes, starting with his triumphs on the battlefield fighting for Scotland, and his subsequent "fall from grace"—till he eventually has sold his soul by killing his king and having others murdered, until he has nothing left at the end but his bravery: which will not allow him to die a coward's death.
First, establish Macbeth's character—as perceived by King and peers. The Sergeant reports to King Duncan and his son Malcolm the brave exploits of Macbeth in battle (I.ii.7), and when Ross enters (48), he continues to give the King glowing reports of Macbeth's valor.
Macbeth meets the witches (sc. iii), and though they deliver predictions for both Macbeth and Banquo, Macbeth is transfixed by what they say, taking it to heart. When Ross and Angus arrive, corroborating the witches' predictions about the title of Cawdor, Macbeth keeps his thoughts to himself, but shows signs of thinking that he might be King (117 and 127).
Lady Macbeth and Macbeth do not speak of Duncan's murder in detail until scene seven. Beginning the scene alone, he worries about their plan. When he shares his thoughts with his wife, she berates and insults him until he agrees.
Macbeth changes more in Act Two, scene one. He approachesBanquo, asking him if he will "have his back" when Macbeth needs him (25). However, Banquo lets Macbeth know (26) that he will as long has he does not compromise his ethics.
As the scene continues, Macbeth sees a "ghostly dagger." When he has killed the King, he starts to fall apart (ii.22). In scene three, the murder is discovered, and Macbeth is cool enough to kill the guards he has framed for the deed (116). In Act Three, we hear Macbeth obsess about killing Duncan, but also how crazy he is because Banquo's "sons" will be kings, and not Macbeth's—he is childless (i.48). Then we learn that Macbeth has convinced men to kill Banquo and his son Fleance (75) by blaming the men's poor circumstances on Banquo (which is a lie). In scene two, Macbeth rants, especially in line 36. He's losing his grip, but making plans without his wife's knowledge (45), further alienating himself from others. In scene four, Macbeth acts like a lunatic, in seeing Banquo's ghost. His descent into madness is evident (50).
Macbeth returns to the witches for more predictions (IV.i.50): here they give him half-truths that convince him that no one can harm him. At the end of scene one, Macbeth plans to murder Macduff's entire family and all that live there (150).
In Act Five, scene five, we learn that the once-strong Lady Macbeth has gone insane in face of all the murders committed at their hands, and has killed herself. Macbeth is virtually unphased (17). In scene seven, Macbeth faces Macduff, still holding onto a fragile hope that the witches told him the truth. He realizes that they have lied (vii.48), but still he bravely fights Macduff to the death (line 56).
Through these examples, it is easy to see how the once valiant Macbeth has sold his soul for power. The only thing that remains is his bravery.