There are several factors that contribute to the death of King Duncan. Macbeths character plays a big part in the crime, being a strong warrior who wants to be constantly in control. His ambition is his main downfall. He is really a coward; he needs the ambition and assertiveness from his wife to push him to do the deed. His wife, Lady Macbeth is very vital to the death of Duncan. She is very cunning and dominant in the relationship. Also Duncans trusting vulnerable state contributes to his murder. The Witches influence on Macbeth is very strong. They give Macbeth the first motive to murder Duncan.
In the opening scene of the play we are introduced to three Witches. The Witches mention the name Macbeth. Without even meeting Macbeth we assume he has a link with the supernatural evil world. "Fair is foul and foul is fair" on the one hand, this contradictory statement is the kind of riddle we would expect from Witches; on the other, the line suggests a paradox that runs throughout the play.
The introduction of Macbeth as a warrior hero is crucial. Phrases such as "Valour's minion" and "Bellona's bridegroom" exemplify Macbeth's super heroism. King Duncan himself praises these things upon Macbeth. He obviously thinks highly of Macbeth and trustingly gives him the title of Thane Of Cawdor although the last person he trusted with this title betrayed him. This shows Duncans personality to be trusting but also rather vulnerable.
In scene 3 the Witches begin by boasting about their recent attacks on a sailors wife. This shows how evil they are and how they attack innocent people for fun. This is exactly how they have targeted Macbeth. When the one witch says Ill give thee wind it proves how powerful they are.
Macbeth's first words "So foul and fair a day I have not seen" ironically recall the Witches' paradoxical phrase "foul is fair" in Scene 1. This is also the moment he meets the Witches for the first time and they hail him as "Thane of Glamis", "Thane of Cawdor", and "king hereafter. Their greeting startles and seems to frighten Macbeth. When Macbeth questions them further, the witches vanish into thin air. Almost as soon as they disappear, Ross and Angus appear with the news that the king has granted Macbeth the title of Thane of Cawdor.
When Macbeth and Banquo discuss this new title Banquo is of the opinion that the title of Thane of Cawdor might tempt Macbeth to seek the crown as well. Macbeth questions why such happy news causes his "seated heart knock at ribs / against the use of nature," and his thoughts turn immediately and with terror to murdering the king in order to fulfil the witches' second prophesy. Already we question Macbeths loyalty to the king.
In scene 4 Duncan refers to his previous Thane Of Cawdor as a gentlemen whom he built an absolute trust. This proves to be quite ironic, it seems that once again vulnerable Duncan is putting all of his trust into a man who could betray him.
Lady Macbeth reads a letter from her husband informing her of his success in battle and, more importantly, of his encounter with the witches, the future he has ahead of him. Immediately her thoughts turn to possible failings in her husband. She fears that his nature is not ruthless enough he's "too full o' the' milk of human kindness to murder Duncan and assure the completion of the witches' prophesy. He has enough ambition she claims, but lacks the courage to act on. Lady Macbeths attitude is strongly reminiscent of the Witches; she talks of pouring her spirits in Macbeths ear like some potion to alter his ego and she invokes the spirit of evil to help her. She is almost ashamed of being a woman unsex me here like it is a weakness.
When Macbeth returns home and tells his wife of Duncans visit, she tells him O never shall sun that morrow see! meaning that after his overnight stay Duncan will not be waking up. She is a very manipulative and you get the impression she is in charge of the relationship.
Macbeth agonizes over whether to kill Duncan, recognizing the act of murdering the king as a terrible sin. He struggles in particular with the idea of murdering a man who trusts and loves him. He would like the king's murder to be over and regrets the fact that he possesses vaulting ambition" without the ruthlessness to actually murder Duncan.
Lady Macbeth immediately detects Macbeth's self-doubt. When Macbeth admits to her that his golden reputation might lose its "gloss," she sets out to strengthen him by mocking his perceived weakness And live a coward in thine own esteem. Finally she tells him that her own lack of pity she would murder her own child as it suckled at her breast. Macbeth is astonished by her cruelty but resigns to follow through with her plans.
In act 2 Macbeth imagines that he sees a bloody dagger pointing toward Duncan's chamber. Frightened by the apparition of a "dagger of the mind," he prays that the earth will "hear not steps" as he completes the awful deed.
Lady Macbeth's opening words introduce new emotional intensity. Fear of failure has been replaced with fear of discovery, and even though she describes herself as drunk with boldness and on fire with passion, she is just as easily alarmed as her husband is by the tiniest noises and movements. Lady Macbeth shows a softer more feminine side when she sates that she would have committed the murder herself, had she not been put off by the idea of the resemblance of the sleeping king to her own father. With her earlier taunt to Macbeth that she would have dashed out the brains of her own child she is showing her first signs of weakness.
When Macbeth returns in much guilt of his deed Will all Neptunes ocean wash this blood? his wives weakness is soon forgotten A little water clears us of this deed: How easy it is then! she doesnt have any regret for the murder they have just committed and once again orders Macbeth to stop feeling so guilty and to shape up to the kingship he is about to receive.
Wake Duncan with thy knocking! I would thou couldst! is Macbeths last line in this scene is leaves the impression that he is so full of guilt he would do anything to turn back time and wake Duncan. This is cowardly personality showing and that he really didnt want to kill Duncan but the ambition of the time overcame him.
In conclusion to this I think the main factor that contributed to King Duncans death was the influence of Lady Macbeth. He devious mind and masculine ways pressurized Macbeth to fully committed to deed. He is just to blame with his ambiguous personality but his wife his is weakness and she gives him that extra push. By taunting him she bullied him into thinking she was the leader in their relationship. Being a woman she wanted to feel the power of a man and this was her way of doing this.