At the beginning of the play, Macbeth is portrayed as noble (though obviously capable of extreme violence.) When we first hear him described, it is as a brave warrior, fighting against Duncan's enemies. At the end of the play, he is a murderous, wicked man, consumed by his own ambition and led astray by the false belief that he cannot be destroyed by a man "not of woman born."
In between, we see a steady evolution. A major turning point is in the final scene of Act I, where is is vacillating over killing Duncan. He is goaded into the murder by his wife, who challenges his masculinity and his honor:
When you durst do it, then you were a man;
And, to be more than what you were, you would
Be so much more the man.
With Duncan's death, though, Macbeth begins to be consumed by ambition and paranoia. He has Banquo murdered, and eventually the family of Macduff. He is torn by guilt over Banquo's death, but by the end of the play, he is not only no longer really guilt-ridden, but pursues his bloody course of action without consulting with his wife. She, in many ways, is the mirror image of her husband. By the final act, it is Lady Macbeth who is wracked by guilt, and seeing visions shortly before her self-inflicted death.