Macbeth's character flaw, in a single word, is greed. (In more than one word, it would be unfettered political ambition.) Greed, by definition, means the selfish desire to acquire something, a desire that surpasses all reason. It is Macbeth's overwhelming, selfish desire to gain the throne and rule Scotland that drives him to murder Duncan. Macbeth violates his own conscience and knowingly sacrifices his soul to wear Duncan's crown. His greed--he would have not some power as Thane of Glamis and Thane of Cawdor, but all power--destroys him. Once Macbeth is crowned, he continues to murder, again and again, to maintain his hold over Scotland. He becomes not just a tyrant but a monster, one who slays the defenseless and the innocent. The witches play a role in Macbeth's destruction in that their prophecies awaken that which is in him: the fatal flaw in his own character.
William Shakespeare's "Macbeth," is one of his greatest tragedies. The play centers around the title character, Macbeth, an ambitious man who stops at noting to get what he wants, and causes ruin for everyone along with his ddownfall.
Early on in Act 1, the witches greet Macbeth as the new "Thane of Cawdor", and later, "King." He initially is startled by these prophecies and questions them in his remark, Why do you dress me in borrowed robes?"
But as soon as that prophecy is fulfilled, he relishes in the thought that he will eventually become King. And it is this ambition, along with the prodding of Lady Macbeth to murder Duncan in his sleep. If he didn't have ambition, he might have perhaps thought that Duncan would have a heart attack or die in battle and name Macbeth his royal heir.
But upon news that Duncan will name his son the royal heir, his ambition is renewed as he realizes that he must take action as he thinks about Duncan's son, "That is a step/On which I must fall down, or else o'erleap,/For in my way it lies."
Macbeth knows he has to do something to make the witches' prophecies come true. And he does so because of his ambition.