This is an excellent question, but hard to answer. The short answer is that he started off as a good guy but turned into a bad guy because of his ambition. That is the kind of answer you might get from a person who considers him a tragic hero. I believe that Shakespeare was trying unsuccessfully to make the audience feel that Macbeth was a hero, or a tragic hero, and not an out-and-out villain like Richard III or Iago. Shakespeare tried to transfer some of the blame for Macbeth's crime to his wife, suggesting that Macbeth didn't really want to kill King Duncan and wouldn't have done it if Lady Macbeth hadn't kept nagging him. Shakespeare also tried to transfer some of the blame to the three witches, suggesting that he wouldn't have committed the crime if they hadn't convinced him that it was inevitable, that he couldn't help himself. But in the long run he still ends up looking like a bad guy. I don't believe the audience ever sympathizes with him, but I believe Shakespeare wanted the audience to sympathize with him. Macbeth is "called" a tragic hero by some critics, but not everyone would agree that he really is a tragic hero.