Ambition in Macbeth is not about the characters' determination to succeed towards a goal. Ambition in the play as a negative character trait: not just a desire to achieve something, but an unnatural desire to achieve something at any cost. Shakespeare has Macbeth speak the lines “vaulting ambition, which o’erleaps itself” in Act I, Scene VII. By choosing the verb “Vaulting” (jumping over), he suggests that to fulfil his ambition to become king, Macbeth must overcome any obstacle that stands in his way. This obstacle is King Duncan, and the only way to remove this obstacle is to murder him.
To murder a king was a shocking, unnatural act in Jacobean England but Macbeth is prepared to commit regicide to realise his ambition. Ultimately, the cost he will pay for his ambition is his own life.