Macbeth was a good man but he made the wrong choices. In Macbeth, what are those wrong choices?
First, whether Macbeth was a "good man" is debatable. Shakespeare may well be arguing that it is through the choices we make that we reveal whether or not we are good. Certainly, deciding to kill Duncan is not evidence of strong moral character.
Essentially, Macbeth is an ambitious character. Initially, his ambition led him to be an outstanding soldier and rise rapidly in the esteem of Duncan and other Scottish soldiers and nobles. When the witches suggest that he could become a king himself, we begin to see a darker side to his character, one where ambition leads him to violate his moral duties of hospitality and loyalty, lie, cast blame on the innocent, and even murder.
While each of these deeds represents a choice, they are not the choices of a good person who makes a mistake of judgment, as one could argue was the case with Othello, a character whose downfall was in part due to being overly trusting of the wrong friend. Macbeth's choices involve being willing to murder to obtain power. Once he has power, he chooses to act as a tyrant rather than as a benevolent leader. These are not choices a fundamentally good person would make.
Macbeth's wrong decisions stem from his choice to not consider the multiple interpretations and implications of the information that he receives. As mentioned above, Macbeth chooses to listen to the witches and believe their prophecy, but he does not consider that there might be multiple interpretations of their words. He foolishly believes that no one can hurt him because he believes that everyone is born from a woman--he does not stop to consider that there might be a different interpretation of the witches' statement. Further, Macbeth does not allow fate to play its course--he is told that he will one day become King, but the witches never say how this will happen. Macbeth does not consider that the statement implies that he could become King by other means, so he rushes the process by murdering Duncan. So, Macbeth's bad decisions boil down to his inability to see all sides of his situation.
He listens to the witches who tell his future completely without thinking of the consequences, chosing murder and mayhem to achieve his future goals rather then letting fate run its course.
He listens to his wife and lets her lust for power overthrow his judgment.
He choses to kill his friend Banquo instead of listening to his valuable advice.
Mainly, one could read Macbeth as a character whose ambition overwhelms his sense of good judgement and morality.