In order to structure your essay I would suggest considering two other factors which, added to Macbeth's ambition, create the tragic events of the play.
As well as Macbeth's ambition, we have to realise that the desires of Lady Macbeth are instrumental in driving her husband forward. She tells us in Act I scene V that Macbeth will need more than just his own ambition to succeed-
Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
Macbeth requires more 'evil' than he currently possesses, and Lady Macbeth is more than keen to supply it-
Hie thee hither,
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round,
Lady Macbeth is instrumental in driving her husband to kill to gain the throne.
We also need to remember that the witches are deliberately persuasive and their methods of engaging Macbeth are strong. He is told of his ascendance to the Thane of Cawdor position by the witches, before the news of Duncan's decision reaches him by messenger. It is presented to him as a natural consequence that he will become king-
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme
As he is already Thane of Glamis, and now Cawdor, he has faith that the third prophesy will also come true, and he chooses not to heed Banquo's warning-
oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence—