Macbeth is essentially a good man, but he makes the wrong choices. Is this true?

I'm not sure that I believe this is true.  Macbeth begins the play as a loyal subject and friend to Duncan.  He defends his country against attacks on two different fronts -- from both a traitorous rebel and a foreign invader -- and he risks his own life to see justice done.  However, it doesn't take much for Macbeth to become ambitious, greedy, self-serving, and violent.  The Weird Sisters tell him that he will become Thane of Cawdor (which he has already, he just doesn't know it yet) and king.  He is shocked to learn that their first "prediction" has come true, and he immediately begins to dream of the second.  

It takes only one scene -- the space required for Duncan to name his older son, Malcolm, heir to the throne -- for Macbeth to begin to scheme against his king and friend (and kinsman).  He says, "Stars, hide your fires; / Let not light see my black and deep desires. / The eye wink at the hand, yet let that be / Which the eye fears, when it is done, to see" (1.4.57-60).  Already, Macbeth is plotting something so foul that he wants the stars to go dark so that no one will be able to see what he's thinking.  Moreover, he doesn't even want his own eye to be able to watch what his hand is going to do; it stands to reason, then, that he's already plotting Duncan's murder.  It doesn't seem to take much or long for Macbeth to become totally corrupt; a good man, it seems to me, could never so completely shake his conscience as Macbeth eventually does.