It is difficult to say that Shakespeare is saying that ambition in and of itself is bad. But it is clear that ambition unchecked by a sense of morality and humanity can be a corrupting influence. As he is agonizing in Act I Scene 7 over whether to proceed with the murder of Duncan, Macbeth admits that there is nothing other than ambition that motivates him to murder Duncan:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other—
This is juxtaposed with the "meek" and "great" way in which Duncan has conducted himself while in office, as well as the morality of killing one's own kinsman and guest. Ambition makes Macbeth susceptible to the exhortations of Lady Macbeth and the machinations of the witches. It causes him to turn his back on what we might call today his "moral compass" and do things that he realizes are fundamentally wrong. Eventually, it destroys both of the Macbeths. It is not just that he is ambitious, but that he embraces ambition over his other virtues.