An act as gruesome as this goes against everything Macbeth stood for and Lady Macbeth herself declared that he was,"too full o" the milk of human kindness", to commit this crime. This is a good explanation for what was to come in the succeeding Act.
His famous soliloquy at the beginning of Act 2 is a deeper level of his tumultuous psychological imbalance. The "dagger speech" as it is commonly referred to, is a manifestation of the inner struggle Macbeth feels as he contemplates murdering Duncan. He is on the verge of murder but his conscience is beginning to get the better of him. This illusionary conflict is a poignant example of the direst extent of Macbeth's personal battle. It is the epitome of shame and guilt and the hallucinatory manifestations, the depths of dementia.
Subsequently after the murder, when recounting the details to Lady Macbeth, his torment is evident. His inability to say "Amen" when he heard one of the guards cry, "God bless us!", psychologically analyzed would read as a physical inability to speak caused by his paralyzing doubt about the correctness of the murder. The inner world of his psyche invades his physical world. His troubled sleep which also results from his, "murdering sleep", is a metaphor of his troubled existence. He has consigned Duncan to eternal rest so he must therefore live in eternal anxiety and torment over his bloody deeds. .
After this, we are only intimately reacquainted with Macbeth in Act 3 when he is pondering the threat that Banquo poses. At this point, Banquo seems to be Macbeth's biggest fear as he is the only other person besides Lady Macbeth who knows about the witches" predictions. Macbeth is insecure that Banquo would make his knowledge known and implicate and hinder his ambitions to become king. Similarly the jealousy and envy that is raging inside him that Banquo shall produce heirs to throne, agitates his psyche to such a heated state that the only solution he can contrive is to murder both Banquo and his son Fleance.