Ultimately, Macbeth's decision to kill the king is driven by his unchecked ambition, although the witches provide him with tempting glimpses of the future, he still has doubts. Even after Lady Macbeth uses all her powers of persuasion, he is still uncertain about whether he should kill Duncan.
It is only after his ambition is stirred that he realizes that he must grasp the moment that is presented, his ambition is the driving force, it is stronger than his loyalty to the king, stronger than his sense of morality, stronger than the power of good that lies within him that warns him Duncan will be mourned, Scotland will weep with great despair at his death.
"Macbeth: Upon the sightless couriers of the air,
Shall blow the horrid deed In every eye,
That tears shall drown the wind. I have no
To prick the sides of my intent, but only
Vaulting ambition, which o'er-Leaps itself And falls on the other.— (Act I, Scene VII)
Once he kills the king, Macbeth becomes drunk with power, he is so addicted to the feeling of being king, that he determines that he will protect his authority by murdering Banquo and trying to have Fleance murdered to eliminate them from interrupting his power.
Macbeth also decides to visit the witches for a second prophecy, he demands to know more about the future, when he is given more visions of what is to come and warnings, he plunges ahead recklessly with a plan to kill Macduff due to the witches warning.
His ambition is still ruling his mind, he sees nothing wrong in eliminiting Macduff who poses a threat, even though he does not have any details about the type of threat. And, because Macduff is not at home when the killers arrive, they murder everyone in the household instead, including women and children.
Macbeth will do anything to feed his addiction to power, to satisfy his need to stay in power, once quenched his ambition thirsts for more power, more authority. Macbeth surrenders all the comforts of being human to be king.
"In Macbeth, ambition conspires with unholy forces to commit evil deeds which, in their turn, generate fear, guilt and still more horrible crimes. Above all, Macbeth is a character study in which not one, but two protagonists (the title character and Lady Macbeth) respond individually and jointly to the psychological burden of their sins."