Despite his own desire to be king, Macbeth still is not certain that he should kill Duncan. He is said to be, "...too full o' th' milk of humane kindness," (Act i sc v) to act on the Witches' prophecies by Lady Macbeth. This is quite a contrast to the way he was described in Act One Scene Two when the Captain talks about his bravery in battle. The difference here is that Macbeth is being convinced to kill his king rather than kill his enemies. Lady Macbeth persists and eventually persuades him to go ahead with the murder, "Away, and mock the time with fairest show, false face must hide what the false heart doth know."
We can see how affected Macbeth is by the event when he sees the vision of a dagger in Act Two Scene One. His mind is playing tricks on him because he is so afraid, "...a false creation proceeding from the heat-oppresed brain?" He is having to choose between his loyalty to the king and gaining a kingdom himself, but in the end his ambition overcomes his conscience and he goes ahead with Duncan's murder, "Whiles I threat he lives: words to the heat of deeds too cold breath gives."
Although the vision of the dagger shows us Macbeth is under a lot of pressure, it doesn't necessarily mean his character is changing or that he is going slightly mad. It does mean, however, that even though he is a bit disturbed by the thought of killing his king, he is still able to take control of his actions.
After the murder things begin to change. Both Macbeth and Lady Macbeth are very distressed and this is shown by the break in rhythm in Act Two Scene Two:
Lady Macbeth: "Did you not speak?" Macbeth: "When? Lady Macbeth: "Now." Macbeth: "As I descended?" Lady Macbeth: "Ay."
Shakespeare often broke up the rhythm of a characters speech to illustrate how emotionally unstable they were. In this case Macbeth and his wife are very nervous and guilty because they have committed the worst crime possible, the murder of a king.
In the time 'Macbeth' was written Guy Fawkes and other conspiritors had decided to plot against King James I and kill him by blowing up Parliament during his speech on November 5th. As a result of this, many plays were written that favoured plotters against the monarchy coming to a bad end. Also, it was believed that disturbing the social hierachy (by, for example, killing a king)everything would be thrown into chaos.
This was something else Shakespeare picked up on so he wrote about such chaos in Act Two Scene Four, "Thou seest the Heavens, as troubled with man's act...by th' clock 'tis day, and yet the dark Night strangles the travelling lamp." In this scene the sun has been obscued and Duncan's horses had eaten each other. Scenes describing happenings such as these would have been totally acceptable to the Jacobean audiences because it agreed with James I's beliefs.
After killing Duncan, Macbeth is crowned the knew king because the original heir (Malcolm) had fled on hearing his father had been murdered. In order to keep his throne, Macbeth has to kill many other people such as Banquo, Fleance and Macduff's family. The other murders are actually commited by hired murderers rather than Macbeth himself. This could mean that he dares not kill anyone directly because of the guilt he would bare. He had already killed the king, perhaps he wanted to avoid going through that experience for a second time.
The point where he plots to kill Banquo is another turning point in Macbeth's character. Banquo was his best friend that he fought alongside in battle and yet he murders him because the Witches prophecied that his sons would be kings. This shows us that keeping the throne is far more important to him than keeping his best friend. First he betrayed the king and now he has betrayed his own best friend. His ambition at this stage is far greater than his loyalty to even his closest companions.
However, Macbeth does not kill Banquo without his conscience coming back to haunt him. In Act Three Scene Four Banquo's ghost appears at a banquet and sits at the king's chair (it is significant that he is sat in the king's chair because his sons were to be the future kings of Scotland). Macbeth's guilt is beginning to catch up with him and shows itself as a ghost. His mind is playing tricks on him again because he has such a huge weight on his mind, the guilt of murdering his king and the guilt of murdering his closest friend.
This guilt is obviously taking it's toll on Macbeth because even Lady Macbeth is keeping some control. He comments on how, "...you can behold such sights, and keep the natural ruby in your cheeks, when mine is blanch'd with fear." Lady Macbeth deals with the situation better than him probably because she did not have to carry out the first murder.
Macbeth's insecurity forces him to pay another visit to the three Witches so that they can tell him his future and give him some peace of mind, "Though castles topple on their warder's heads...answer me to what I ask you." (Act iv sc i) The Witches then call the Three Apparitions to give Macbeth the advice it wants to hear so badly. The first thing he is told is to, "Beware Macduff", the second that, "...none of woman born shall harm Macbeth", and finally that, "Macbeth shall never vanquish'd be, until Great Birnam Wood, to high Dunsinane Hill shall come against him."
These words were exactly what Macbeth needed and wanted to hear, but probably did him more harm than good. They filled him with false hope and confidence, but still reminded him that Banquo's sons were to be the future kings of Scotland and not his own descendants.
Nevertheless, Macbeth takes heed of the first piece of advice and sets out to kill Macduff and his family, "The Castle of Macduff, I will surprise...give to th' edge o' th' sword his wife, his babes." (Act iv sc i) Macbeth is still focused on keeping his throne and is prepared to kill a whole family if needs be. This is also another turning point in his character, it shows us that he is willing to do anything to get peace of mind and keep the crown and does not care what sacrifices he makes to get there. He feels that having chosen an evil path he cannot turn back, "I am in blood, stepped in so far that should I wade no more."
When he hears of his wife's death he feels his life has lost meaning and what he desired so much had no point because he had lost the woman he loved, "There would have been a time for such a word: to-morrow, and to-morrow, and to-morrow."
At this point more and more things start to go wrong. Birnam Wood is sighted marching towards Dunsinane Castle which was, as the Apparitions said, the sign that he would be vanquished. Then, the last of his certainties disappears when he is told Macduff was not born of woman but by Caesarean section. This is where Macbeth's character changes for the last time. He is, in a sense, returning to the person he was at the beginning of the play. He refuses to surrender even when there was no way he could win the battle. In desparation he fights bravely with Macduff untill he meets his unfortunate end, "Yet I will try the last. Before my body, I throw my warlike shield. Lay on Macduff." (Act v sc vii)
