There's many perspectives from which to view ambition in the play Macbeth: from both then and now, and from Macbeth's and Lady Macbeth's viewpoints.
During Duncan's regin, there really was no ambition, not ethical or otherwise. Ambition was negated because society was regulated and ordered, like the numbers on a clock-face. The Diving Right of Kings and the Great Chain of Being said that the King was King and a Thane was a Thane: each member of society was fixed in his role. The social strata was engineered for no upward mobility; it was designed to protect "God's holy vessel" and insure allegiance. But, there was one loop-hole: murder.
Macbeth knows he really could never have been King, even though he was promoted to the highest ranking Thane. The chances of Macbeth outliving Duncan were good, sure, but there's Malcolm and Donalbain to "o'er leap." So, the only ambition Macbeth has is of the malevolent and Machiavellian kind: murder by way of "the ends justify the means." He knows if he wants to be King, he will have to murder Duncan and blame it on the two sons. He will have to subvert natural order and time: foul will have to become fair. This, obviously, is a violation of the ethical realm for that time. In effect, Macbeth is selling his soul to be King: he knows he will got to hell for it. But, what if there's no hell? Anything is permitted.
Today, we look at Machiavellian politics more favorably, and we are not so moralistic to condone murder if the rewards are great. Today, a historian might say that Macbeth staged a coup, or that Duncan's reign was not strong enough to withstand rebellious and traitorous thanes. Maybe Macbeth was rebelling against the unfair social stratifications and the Divine Right of Kings. It is easy to cast off the ethical when looking back on history.
Ambition is more interesting from Lady Macbeth's perspective. Even though Macbeth's ambition is limited by the hierarchies of society, at least he has a chance to be King. She has none. Ambition is a male enterprise entirely. She cannot even kill Duncan: he looks too much like her father.
Lady Macbeth tries to cast off her femininity and assume the cruel nature of male warriors in her famous soliloquy, but she can only gain status through the male, her husband. Ambition, therefore, is vicarious for her.