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.