For me, the main source of evil as presented by Shakespeare is internal - it comes from within Macbeth himself but is fed and watered by external circumstances – i.e. the witches' prophecies and his wife's pushiness. It seems clear from the outset that Macbeth's personal ambitions and driven nature have led him to great success in battle and in political terms. The way his curiosity and interest is aroused by the witches suggests to us that he has previously harboured secret and dangerous ambitions known to none but himself (and I believe the text supports this - see Banquo's comment on Macbeth's reaction to the witches' prophecies on their first encounter upon the heath). But at this stage, although his interest is aroused, I don't believe we are looking at a murderer because, as we see later on in that scene, Macbeth is struggling with the implications of the news that he has been appointed Thane of Cawdor and he debates with himself whether these prophecies are good or evil, concluding that he may leave the outcome of the final prophecy to chance, 'Without my stir' as it was chance or fate that landed him the seat of Cawdor. Macbeth is very aware of the moral consequences of killing the King, and is also sufficiently self-aware of what drives him, his “Vaulting ambition, which o'erleaps itself/And falls on the other”, in other words, he recognises that it's his shallow ambition that threatens to overwhelm his moral judgement and everything else. He contrasts this with the King's goodness and virtues and has a vision of heaven's reaction to the deed he is contemplating. The contrast couldn't be greater - Macbeth's imagination is carrying him away and he is almost talking himself out of the act but then at that precise and critical moment, Lady MacBeth makes her entrance and proceeds to give him a talking to! The dramatic timing couldn't be more significant here. What is real and what is fantasy that make this play such a fascinating one. I think Banquo's words in Act 1 Sc 3 sum up brilliantly this fascination of Shakespeare with the relationship between evil forces and human nature in the play:
“And oftentimes, to win us to our harm/The instruments of darkness tell us truths/Win us with honest trifles, to betray's/In deepest consequence.”
I believe that Macbeth is ambitious, and I think that before he meets the weyard sisters he has hopes of succeeding Duncan. Scotland was an elective monarchy, like Hamlet's Denmark. That's why Claudius became King not young Hamlet. Macbeth can reasonably hope, as a member of the Royal family (which he was) that he would get the vote over young Malcolm. So he gets a sort of confirmation of this hope from the sisters. He is then greeted by Ross with the words:
"for an earnest of a greater honour, He bade me, from him, call thee thane of Cawdor"
What can that greater honour be? Perhaps Duncan is about to pronounce his view on the succession? And that is exactly what he does, but to Macbeth's obvious disappointment Duncan names Malcolm as Prince of Cumberland, equivalent to Prince of Wales in Scottish terms. And in an elective monarchy, you do not 'name' your successor, the Thanes elect him. As Macbeth says bitterly:
"The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies."
Waiting for old Duncan to fall off his perch is one thing, the chances that Macbeth will outlive young Malcolm quite another.
It is always interesting to see how different productions portrayal the relationship between Lady Macbeth and Macbeth, which one is strong and takes the lead in the murder. I have seen versions where Macbeth seems weak and needs Lady Macbeth to push him into the murder. This has always seemed at odds with the harsh treatment he dishes out to Banquo, and especially Lady Macbeth. However, these murders are carried out by others on his behalf, he does not have to get his hands dirty, and perhaps that is why he needs a push to murder Duncan himself. Lady Macbeth's character seems more complex, her descent into madness seems more sudden. This happens off stage, we don't see her meeting with witches, or even having the conversations with others, therefore the change in her comes as a greater shock to us.
The play is wonderfully open to interpretation in performance with directors and actors able to put their own ideas on culpability. Macbeth starts off as a man who has everything going for him - a lauded warrior who seems to have been widely admired but once the Weird Sisters set him on the path, he turns murderous. He seems aware of the depth of his degradation as a human being but is unable to move off the path he is following. And he loses everything on the way down. The psychology is complex and there doesn't seem to be a simple single answer to Macbeth's downfall.
Whether it's hamartia or not, Macbeth makes the choices he wants, because it suits him. He procrastinates enough in Act I against the initial murder, but his dagger soliloquy portrays his decision to go through with it. He reads the "air-drawn dagger" as an inevitability almost. The blood on the blade in his imagination almost suggest that the deed has already happened, in his mind at least, and by the end of that soliloquy journey, Macbeth is resolute. At any time, he could have refused to kill Duncan; he could have woken him and betrayed his wife by explaining the plan. He doesn't. The idea of being King suits his own ends, and he clearly thinks he can cheat the witches' prophecies of warnings regarding Banquo. He is more a man of action than thought when we meet him as a soldier, but once the Thane of Cawdor prophecy comes true, it is no great leap for him to assume he can be King. I strongly dislike blaming other characters for Macbeth's fall. Lady Macbeth has it right from the beginning--"Art not without ambition" and "wouldst wrongly win." Macbeth wants it all--the question is what it takes to get him moving. So a slight suggestion from some random maybe-women and a short nag session from the wife, and he's off to kill the king? And Banquo? And some poor lady he probably hasn't met and her little kids? Come on. He just wants to hear that his ambition is justified. Once he gets that, he's off. And the ambition, like any other unchecked desire, leads to evil because it overrides his morality. By the way, the only magic that we are actually told that the sisters can do is deprive people of sleep. So they can't really make Macbeth do anything. And, like Romeo, if you really believe in fate, why do you have to do something to move it along?
If the Macbeths were pure evil from the start the play wouldn't be tragic at all, because we'd be happy to see them suffering and defeated and getting what they deserve. I feel the true tragedy lies in the fact that in different circumstances the Macbeths had the potential to be excellent rulers - they both seem to have been well-liked and respected figures at the start of the play, and they're certainly both shrewd and capable - and things could have gone much better for them and Scotland (if they had just been patient, they probably would have ended up the rulers of Scotland without having to kill anyone, the same way Macbeth was made Thane of Cawdor without having to do anything - the prophecy fulfilled itself). They both struggle internally between good and evil (Macbeth swings back and forth between 'I'm going to kill Duncan' and 'I'm not going to kill him because he's a good king and I have no reason to' and later Lady Macbeth is racked with guilt over what they've done).
We're human beings and we all must choose our own course. Ultimately Macbeth chooses evil. And ultimately he has to be held responsible for that choice.
I was re-watching the Nunn/Dench/McKellen version and really noticed for the first time that Macbeth is the one who thinks up the idea of blaming the murder on the servants. He loses his nerve after killing Duncan and she ends up smearing the servants with the blood. Some have suggested that he is being the protective husband by keeping her out of the murder of Banquo. I am more inclined to think he wants to limit the number of people in the know as he starts to add cunning to ambition as a driving force for his actions. So he kills the servants before they can protest their innocence. As I recall he has not shared with Lady Macbeth the prophecy concerning Banquo, certainly not in the letter and there is no hint of it off stage. I have never been able to see Macbeth as a noble soul led astray by evil forces and a wicked woman. What can I say? I have always maintained that what we see and derive from reading Shakespeare depends upon what is in us - our own nature, intellect and mood. To see Shakespeare dramatised upon a stage is no different except that there is a fuller opportunity for audience joining in in its presentation; something that Shakespeare was very good at providing for, although its result always depends upon the quality of the film director, in this case Trevor Nunn. My definite Macbeth version.
So in summary (I could get carried away and write an essay...) I think Macbeth is influenced by the witches and their prophecies - whom I believe to be real as opposed to a figment of Macbeth and Banquo's imaginations. The reality of the supernatural world is extremely strong throughout the play and most people would have believed in it. But it is the interplay between the external forces and the internal motivations, and the power of the imagination and its ability to blur the distinctions between natural and supernatural evil.
NB: All pictures taken by me directly from the film.