In reference to the play 'Macbeth' by William Shakespeare, there has always been great debate as to whether Macbeth is actually mad or just bad. My own take on the play is that he chooses to take the evil path due to a psychological imbalance - a predisposition to duress. Some see the duress as coming from Lady Macbeth, others from the witches. I see influences coming from both of these directions. If Macbeth is mad, then he may also have a predisposition to 'suggestibility.' This often happens when people feel they have no control early on. So, the witches just have to plant the ugly but tempting seed of ambition- and Lady Macbeth's vanity and Macbeth's psychopathic tendencies will do the rest. Outwardly the motive looks like power.
I would argue that it is a combination of things that does this.
First, I would start with the idea that Macbeth is ambitious. If he were not ambitious, nothing else would have mattered. However, his ambitions did not send him along the path to evil without help. Instead:
Second, the witches' prophecy gave him reason to believe that his ambitions could be realized. Before that, he had ambitions, but they were under the surface. Now, he sees an opportunity and that makes his ambition wake up, as it were.
Third, his wife pushes him. She's ambitious too and she thinks he is not ruthless enough.
So it's a combination of those three.