What does the play Macbeth by William Shakespeare suggest about the significance of the individual's response to threatening forces?

At the start of the play, we learn of Macbeth's bravery on the battlefield. Duncan praises him for this and this indicates that Macbeth is a loyal Thane of the king. However, Macbeth proves himself to be morally weak and easily persuaded to engage in murderous deeds in pursuit of his own gains. How much is this is due to his individual weakness and how much is it the power of external influences? 

There will always be debates about the witches themselves. Are they real or hallucinations? Do they merely suggest things to Macbeth or do they actually foresee things that are going to happen? In other words, do they simply present the possibility that Macbeth could become king (thereby leaving it up to him) or do they actually foresee what he is fated going to do? Or, if they are supernatural, do they unfairly cast a spell on Macbeth in this way and is it therefore not his fault? Is he at the mercy of their powerful influence? 

These questions can not be definitively answered. And this is useful for a discussion about the individual reacting to external influences as well as his own internal "voices." If Macbeth is reacting to real witches (forget for the moment that they have a supernatural advantage over him), this shows how the individual can be pressured and/or persuaded by others, by the culture, or any external influence. This is especially evident when the individual is persuaded with the promise of some kind of reward. If the witches are hallucinations, representing Macbeth's own desires and impulses, it shows how he (subconsciously) is blaming or attributing his own weakness on the external world. So, even in this case, Macbeth is reacting to what he thinks is the influence of the external world. 

Macbeth is also manipulated by his wife. This dynamic is the subject of debate as well. How much can we blame her? How much should we blame Macbeth for giving in to her malicious encouragement? 

The play suggests that this particular individual, Macbeth, is easily persuaded to commit horrible things in pursuit of selfish goals. It suggests one of two things, or perhaps a combination of the two: the individual is weak and quickly bends when faced with threatening influences and/or threatening influences in the external world are quite powerful and difficult for anyone to resist. 

Consider the parallels between the witches and our contemporary external influences. How do the witches compare with advertising and how we are persuaded to buy into things? How does political ideology make us act, vote, and behave in certain ways? How much does it show individual weakness? How much does it show the power of threatening or pervasive influence?