How does Macbeth go from good to bad?
Macbeth's fall from grace or goodness happens from a combination of events converging at once. On the exterior, Macbeth is a good and loyal subject of King Duncan, bravely fighting to defend king and country. But deep inside, way down, he harbors a secret ambition to possess power. This deeply hidden ambition is awakened by the prophecy of the three witches in the beginning of the play.
Once Macbeth is given a glimpse of the future, and the prediction that he will sit on the throne of Scotland, the temptation to acquire this power immediately becomes overwhelming.
After Macbeth is made Thane of Cawdor by the king, he realizes that his destiny includes becoming King of Scotland. However, he wants it now. Once he tells Lady Macbeth of the prophecy, she begs and pleads with her husband to seize the opportunity and murder the King when he visits their home that evening.
Once Macbeth crosses to the dark side and becomes comfortable with murder. He murders again and again in order to protect and preserve his power.
His goodness sacrificed as an offering to attain power and to keep it. He trades his humanity for the crown, his soul for the glory that comes with being king, even if it is for only a brief time.