Macbeth as a doppelgangerWhat message about humanity can we ascertain from comparing Macbeth as a doppelganger with other characters?
I think one simple truth about humanity is that most people are ambitious. Macbeth wanted everything, and he didn't want to wait for it (or his wife didn't). When you compare Macbeth and his wife, you can see that both were ambitious but Macbeth was not as proactive, until his wife spurred him on.
Macbeth, in fact, was ambitious but he did not posses evil nature in him. He was instigated by his wife and he was affected by her vices. Therefore, he saw a floating dagger infront of him when went to murder king Duncan. Because his conscience never allowed him to do so. He was satisfied with the position he attained by dint of his hard work, sincerity, loyality. His wife was ambitious by nature and that is why her conscience allowed her to sort out a plan to murder King Duncan before his arrival.
From the above point of view, we can see that people who affect vices of others and act against own conscience, are bound to perish at the end.