Macbeth becomes a monster. He wants power, for inexplicable reasons. He will do anything to get that power, and then he will do anything to hang onto it. He murdered his cousin Duncan and his best friend Banquo!
Sheer ambition for ambition's sake is not a good thing. As in Macbeth's case, it is his fatal flaw...that and his gullibility to believe that the witches would actually tell him the truth with no tricks up their sleeves. Ambition destroys the Macbeth family--Lady Macbeth pushes her husband toward the throne and then she goes mad with the guilt of the methods they used to get there. Her suicide is a prime example of what ambition can do. Macbeth also suffers, but the other way. He becomes even more bloodthristy and seeks out others to murder in order to secure his throne.
The moral issue behind the whole play would be guilt. The two main characters in this play, Macbeth and Lady Macbeth, are both very ambitious, yet in different ways. Both show guilt, but they show it at different times of the play. In the beginning act, Macbeth feels guilt for what he and his wife have planned. He does not want to go through with killing Duncan. In act I, scene VII Macbeth shows his indecision in his soliloquy.
Then Lady Macbeth threatens his manhood and talks him into it by dangling the proverbial carrot (of being king) in front of him. He acquiesces and they do "the deed."
After he is crowned as king, Macbeth's ambition grows. He loses all sense of guilt. It is Lady Macbeth who slowly digresses in her guilt. The famous sleep-walking scene shows how far her guilt has taken her. Then she eventually dies. Macbeth's guilt is never seen after he becomes king. He turns into a paranoid killing machine.