The tragedy of Macbeth unfolds with the murder of King Duncan, setting the story forward with the gradual moral degradation of Macbeth. While external factors and supernatural elements undeniably influence Macbeth’s choice, it is compelling to argue that Macbeth undertakes the most responsibility for Duncan’s death. Examining the character himself, Macbeth is regarded as heavily responsible for his overwhelming ambition for power, susceptibility to manipulation, and lingering guilt that haunts him. The three indications of Macbeth bear him the primary responsibility for the murder. 

First, Macbeth's ambition is a driving force that leads him toward the murder of Duncan. As the story commences, the plot is propelled by Macbeth’s encounter with three prophetical witches who foretell his fate as Thane of Cawdor, and subsequently King of Scotland. Following the prophecies, Macbeth reveals his ambition for the throne as he describes his ascent to the Thane of Cawdor, corresponding to the words of the witches, as “happy prologues to the swelling act of the imperial theme” (1.3.132-133). Moreover, he also ponders why “chance may crown [him] without [his] stir” (1.3.147-149). The contemplation of Macbeth shows his initial desire for the throne and the joy he derives from fetching power. As Malcolm is declared to be heir of King Duncan, Macbeth’s ambition is intensified as he regards Prince Malcolm as “a step on which [he] must fall down, or else o'erleap”, and later resolves to kill Malcolm and Duncan while wishing his “black and deep desires” are not seen by light (1.4.50-53). As early as before the arrival of Duncan in Macbeth’s castle, Macbeth already shows his craving for power portrayed by himself as “black” and “deep”, suggesting a darker inner side of Macbeth driven by power-hunger. Continuing, his yearning for power intensifies with the prospect of eliminating any obstacles on his path to the throne. Consumed by the idea of securing his throne, Macbeth contemplates the idea of killing Banquo as his prophetic revelation suggests Banquo being the future father of kings. He acknowledges his fear when he comments that to be king “is nothing, but to be safely thus” (3.1.50). Macbeth’s relentless pursuit of power, shown in his determination to act,  indicates his active participation and responsibility in the murder of Duncan. 

Second, while possessing a determined and eager mind, Macbeth is not impervious to manipulation. Throughout the story, Lady Macbeth plays a crucial role in pushing Macbeth towards the murder, and Macbeth proves to be easily influenced by a lack of firm will. Prior to the crime, Macbeth hesitates his choice as he sees Duncan as a king with virtues “plead like angels” (1.7.19). Associating Duncan with the angel, Macbeth struggles against his conscience and loyalty. Nonetheless, when Macbeth rejects the ruse of Lady Macbeth, her persuasive tactics, such as questioning his courage and manhood, swiftly diminish any resistance from Macbeth. Defeated by her accusations, Macbeth again reveals his vulnerability. As Lady Macbeth continues her cunning plan, Macbeth praises her fearless spirit and actively joins her scheme by proposing to frame the chamberlains through “[marking] with blood those sleepy two of his own chamber and [using] their very daggers” (1.7.75-77). Shown in his quick compliance, Macbeth's susceptibility is evident as he appears to rely on Lady Macbeth to guide him through the immoral act. Subsequently, Macbeth changes his mind shortly and claims to be “settled and bend each corporal agent for this terrible feat” (1.7.78-79). Easily wavering his standpoint, Macbeth is to be most guilty for forfeiting his conscience and falling deep into the plot of Lady Macbeth. 

The aftermath of the murder tortures Macbeth and his conscience by putting him through immense feelings of guilt, which further proves Macbeth’s primary responsibility for the murder. As he has just done the deed, Macbeth is so shocked by fear and horror of killing that he immediately starts repenting the crime by questioning if  “all great Neptune’s ocean wash this blood clean from [his] hand” (2.2.60-61). By alluding to the god of the ocean Neptune, Macbeth stresses his remorse, claiming that even with the power of the deity, his sin will not be cleaned. The symbol of blood, introduced in this sentence, becomes a representation of guilt and continuously haunts Macbeth. As the story proceeds, Macbeth experiences a gradual mental devastation of constant hallucinations. Meanwhile, he also undergoes a growing desire for power that drives him to secure his throne by more murders. As he plots the killing of Banquo, Macbeth describes his mind as “full of scorpions” (3.2.38), emphasizing his internal vice that leads to rapid murders. The imagery of scorpions in his mind portrays Macbeth’s perception of his guilt, presenting a haunting and sinful scene. Troubled with this guilt, Macbeth again speaks of the symbol of blood by admitting to Lady Macbeth that he has “in blood stepped in so far” and should “wade no more” (3.4.143). By talking of blood, Macbeth refers to the lasting guilt he has sunk into deeply, acknowledging the irreversibility of his actions. These moments where Macbeth speaks painfully of his sin imply the undeniable responsibility of Macbeth in Duncan’s murder, as the aftermath places its influence on Macbeth profoundly. 

In conclusion, Macbeth’s distinct ambition shown both before and after the murder, his susceptibility to external influences, and his moral remorse emphasized by the symbol of blood, collectively place Macbeth as the central figure accountable for the murder of Duncan. The psychological struggles of Macbeth add to the complexity of the character, presenting him as a man whose personal choices and moral conflicts. Macbeth’s character offers a timeless reflection of the fragility of human morals and the consequences of unchecked ambitions.