This question is debatable, and is one of the central questions of the play. First, one might argue that this is because he is fated to be evil. The three witches who manipulate Macbeth might be compared with the three Fates of Greek mythology, who determined the ultimate outcome of every human. These "weird sisters" are also linked with the Anglo-Saxon notion of "wyrd," or fate, whose grasp no one can escape.
Another possibility is that Macbeth chooses his path out of pride, which results from his ambition. Towards the end of the play, he compares himself to a bear that has been tied to a stake, ultimately to be killed by hounds. He knows that he is going to lose, and to die, but he resolves to "try the last," to fight anyway. This same pride which causes him to resign himself to his demise has also driven him to commit one unspeakable act of evil after another.
MacBeth knows his actions are evil, yet continues to do so because he is victim to "blind ambition which o'er looks itself" that results in his bloody demise. Furthermore, MacBeth is first and foremost a soldier and soldiers are trained to fight unto the death. MacBeth begins his evil tirade, wavers at first, but then is convinced by Lady MacBeth to stay the course and persevere in this business, since they are already steeped in blood so far. When Lady MacBeth commits suicide, he is so far into the business and so morally diseased that he cannot feel anything and chooses just to soldier onwards. By the time he figures out the confusing prophecies of the three weird sisters, it is too late and nothing can really save him. He becomes, ironically, like the former thane of cawdor where "nothing became his life" more than the "leaving of it."
The story of Macbeth in Shakespeare's play is a study in a self-divided personality : both fair & foul,as the 'weird sisters' suggest in their sinister chiasmus in the opening scene :' Fair is foul and foul is fair'. Shakespeare centres upon the theatre in the soul of Macbeth : a soul which suffers from a conflict between his soaring ambition and his imaginative conscience. This intense inner conflict is wonderfully recorded in his soliloquy at the beginning of Act1 Sc.7. It is his ambition empowered by Lady Macbeth's chastisement which compels Macbeth to kill King Duncan to 'catch the nearest way' to the throne.In order to ensure his safety and the security of his crown, he then goes on killing Banquo and others.But his imagination and moral conscience remain awake deep within his self to remind him of retributive justice. Macbeth is a complex study of an exceptional mind, split between good and evil.