Macbeth is a tragedy, and so by definition, the play will dramatize the protagonist's tragic fall. Macbeth is a flawed man who is easily persuaded by the witches and by Lady Macbeth and who is excessively ambition. These flaws lead him to make a series of decisions that eventually end his reign as King of Scotland and his life.
It is important to remember that the tragedies that befall Macbeth are a result of his own actions. While it could be argued that Macbeth is tricked or manipulated by the witches, and thus that coming across them on the heath in Act I is a "tragic" event that happens to Macbeth, it is ultimately Macbeth's decision to believe their predictions. This belief leads him to plan, with Lady Macbeth, to murder King Duncan and take his throne. Over the course of the next few Acts, Macbeth goes on to kill Banquo and Macduff's family, along with many others whom he slaughters in battle. As a result of his actions, though, Macbeth tragically loses his sanity. He becomes paranoid and increasingly ruthless. His ambition makes him a lesser person.
Further, as a result of his greed and paranoia, Macbeth kills his best friend, which also means that he loses (by his own deed, of course) his closest confidant besides his wife. As he continues to plot more murders, Macbeth works alone, leaving his wife out of the plan. As a result, he loses that support system that he had earlier in the play. Lady Macbeth begins to go insane and eventually kills herself as a result of the guilt she feels at what she and her husband have done. At the end of the play, Macbeth is killed by Macduff, thus ending his reign and his life simultaneously. Macduff brings the head of Macbeth to Malcolm, the new and rightful king, as proof and also as an example to the people of Scotland of what happens to a tyrant. Macbeth's reputation will forever be that of a ruthless, traitorous murderer and a corrupt king. The tragedy is that a character who starts out as a respectable and loyal nobleman could rise so high only to fall so far.