The definition of tragedy is a dark event which involves the participation (whether willingly or not, conscious or not) of a character that is admired by many for his qualities but who falls miserably due to a serious fundamental flaw which obscures his best judgment. In the case of Macbeth, it is obvious that his fundamental flaw is ambition that is excessive, immoderate and blinding.
Indeed, had he just allowed event to unfold without forcing them, he would have eventually become king out of his own merit as he is described as a courageous, strong and cunning warrior and leader. However, he is so obnubilated with his ambition that he heeds the ill advice of his very ambitious wife and triggers the series of event that will culminate in his own destruction as well as his wife’s.