While Macbeth is the hand that wields the dagger, Lady Macbeth bears the primary responsibility for King Duncan's death. Her relentless ambition, coupled with a masterful manipulation of her husband's insecurities, directly precipitates the regicide. From the moment she reads Macbeth's letter detailing the witches' prophecy, she envisions Duncan's demise not as a possibility but as a necessity for their ascent. Her subsequent actions transform this vision into a brutal reality, far exceeding Macbeth's initial hesitations.
Lady Macbeth’s ambition is immediately apparent and arguably more fervent than Macbeth’s. Upon hearing of the prophecy, she cries, "Come, you spirits / That tend on mortal thoughts, unsex me here, / And fill me from the crown to the toe top-full / Of direst cruelty!" (1.5.40-43). This plea reveals a conscious embrace of evil, a desire to shed all feminine weakness to achieve their shared goal of kingship. She sees Duncan's existence as an obstacle, a tangible barrier to the crown that Macbeth, though powerful, is too hesitant to remove. Her immediate framing of the situation as one requiring decisive action, contrasting with Macbeth's initial shock and moral qualms, places the onus of plotting squarely on her shoulders.
Her most potent weapon is her psychological manipulation of Macbeth. When he expresses doubts and fears about the murder, Lady Macbeth doesn't appeal to his ambition directly but attacks his manhood. She calls him a coward, questioning his love and courage: "When you durst do it, then you were a man; / ... Art thou afeard / To be the same in thine own act and valour / As thou art in desire?" (1.7.49-54). This is a deliberate and cruel tactic designed to shame him into action. She knows his deepest insecurity lies in his perception of himself as a man, and she exploits this vulnerability with chilling precision. By questioning his courage and fidelity, she forces him to prove his mettle through the ultimate act of violence, effectively coercing him into becoming the murderer she desires.
Furthermore, Lady Macbeth takes charge of the practical execution of the murder plan. It is she who drugs the guards, ensuring their incapacitation. She is the one who places the bloody daggers into Macbeth's trembling hand after he has committed the deed, urging him to "Go get some water, / And wash this filthy witness from your hands" (2.2.46-47). Her composure in the immediate aftermath starkly contrasts with Macbeth's unraveling. While Macbeth is consumed by guilt, seeing spectral visions and hearing phantom voices, Lady Macbeth maintains a façade of control, even chiding him for his distress. This active involvement, from planning to disposal of evidence, demonstrates her deep-seated commitment to the regicide, making her far more than a passive instigator.
Ultimately, the tragedy of Duncan’s death is inextricably linked to Lady Macbeth’s ambition and her ruthless methods. She fuels Macbeth’s latent desires, pushes him past his moral boundaries, and orchestrates the critical steps of the murder. While Macbeth’s hand delivers the fatal blow, his agency is profoundly compromised by his wife’s powerful influence. Her descent into madness and eventual death, haunted by the blood she cannot wash away, serves as a grim testament to the corrosive nature of her ambition and the immense guilt she carries, confirming her as the true architect of Duncan's tragic end.