I believe that Ambition is not the only danger in the play but it is something that is shown as a threat in many ways throughout.
Ambition is shown to be more dangerous than the other potential causes of aggression in the play. Shakespeare shows that the ambition Macbeth has could be dangerous through the simile: "what seem'd corporal melted as breath into the wind" . this suggests that the witches are not the danger themselves because they cannot force Macbeth anything and they are more like a catalyst. However, there must be something driving Macbeth to make the decisions he does throughout the play. This could be his ambition.
The adjective "corporal" could be describing the witches and the fact that they seemed real and there, or it could be describing what they had said to Banquo and Macbeth and how it may only be as real as Macbeth and banjo make it and that what happens depends on whether Macbeth decides to act on what had been said and the ambition that seems to control him.
The king of England at the time would have been part of the audience. He believed strongly that witches were very dangerous and so Shakespeare may have intended to show them and their danger to please the king. However, he may also have meant for this to be a warning for the king and for everyone else watching about how dangerous and how much of a threat ambition can be.