Although, he doesn't easily make that decision. At first, he goes against it, but, with the knowledge that he could be king, he could not help himself from considering it. In Act 1, Scene 7, Lady Macbeth says to Macbeth, "And live a coward in thine own esteem" which will make him want to change his mind into killing Duncan so he prove he is a man instead of a coward. After the constant incite with Lady Macbeth, she and Macbeth made their decision to, "Soundly invite him-his two chamberlains will I with wine and wassail so convince, that memory, the warder of the brain, shall be a fume, and the receipt of reason a limbeck only: when in swinish sleep their drenched natures lie as in a death, what cannot you and I perform upon the" unguarded Duncan." In other words, Lady Macbeth will load Duncan's attendants with liquor, and then, on Lady Macbeth's signal, Macbeth would creep into Duncan's chamber and slay him with his servant's weapons. This act increases Macbeth forward into the path of destruction. After Duncan is found dead, Macbeth murders Duncan's servants so they cannot witness against him. Macbeth's greed has taken control of him and he could not turn back. It only takes that one idea embedded into Macbeth's head to lead him toward corruption.
Now that the witches have succeeded in bringing out Macbeth's evil qualities, they are ready to finish their machinate to Macbeth's downfall. After Macbeth grows more sinful and overpowered with greed he does not make any real attempt to change, and this bothers his conscious. Slowly Macbeth loses grasp of his sanity and self-control. Macbeth's malevolence and deceptiveness are shown further when he becomes so obsessed with the witches prophesies to his friend, Banquo, that he decides to hire two men to kill him and his son. It is not long before Macbeth's own ruthlessness begins to disturb him, greatly. He suffers from troubled sleep, nightmares and loss of appetite.