Macbeth acted as he did because he was ambitious, and his ambition led to his demise. He brought it on himself. He did not have to accept the witches’ prophecies, and decide that he had to be king because they said so. His tragic flaw was ambition.
The tragedy in Macbeth is caused by the hunger for power and the inability to see the consequences of a violent, evil deed. Macbeth, spurred on by his wife Lady Macbeth, just has to be king, once he hears the witches prophecy. The idea, although it was not his goal beforehand, takes over his entire being.
Then, he completely fails to imagine what awful consequences will follow his evil deeds.
"Vaulting ambition" and paranoia are responsible for Macbeth's tragedy. That these two qualities drive Macbeth to his end are so completely realistic is evidenced in the real lives of many a great man. One modern Macbeth is Richard M. Nixon whose ambition for greatness kept him in the political area after his defeat in several elections. But, because he had lost in previous elections, Nixon became paranoic and feared losing even when he was Gallup Poll points ahead. Hence, the debacle of Watergate.
Macbeth tempts the fates by accelerating the predictions of the three witches in Shakespeare's play; then, because he acted out of time, his fears begin and he feels that he must eliminate any contenders for his power.
The influence of the witches is important, but as others have said, it is Macbeth's ambition that is ultimately responsible for his downfall. The witches merely prophesy what is to come, it is up to Macbeth (and, to some extent, his wife) to chart his path to the crown. Later, his ambition even leaves his wife behind, and we see that she is guilt-ridden and a shell of her former self before her suicide late in the play. Hecate also says at one point that she will act to make Macbeth overconfident through the visions that appear to him in the witches' cave, but it is still up to Macbeth to choose his own fate. Obviously, the interaction between fate and free will is one of the central themes of the play.
I, too, believe that Macbeth's ambition was the thing which lead to his demise. That said, the influence of both the witches and his wife had something to do with it. Initially, Macbeth was okay with allowing fate to determine his future ("If chance will have me king, why, chance may crown me. Without my stir."). It was through his own wife's belittling which forced his hand.
Macbeth's ambition definitely leads to his undoing. Post #2 makes a good point about him lacking the moral character to make better choices. Macbeth's ambition did not necessarily have to be a bad characteristic, but Macbeth repeatedly chooses the 'easy fix' or quick solution to his problems. Could he have eventually become king in some other more honorable way? Perhaps the witches' prophecies could have still come true, but through alternate means. Macbeth is a tragedy, because of Macbeth lets his ambition override his morals, loyalty, and common sense.
I believe the strongest argument for what caused the tragedy in Macbeth is offered above. To offer another idea, we might look at the role responsibility plays in the death of Duncan.
Macbeth doesn't come up with the idea for murder by himself but is persuaded by both his wife and the witches. For this reason he is able to say yes to the murder without, initially, being responsible for the act.
Later he finds that he is the only person who will be held responsible for Duncan's murder (and Banquo's murder), as the witches are not liable really and Lady Macbeth is dead.
The tragedy, seen in this light, comes in the fact that Macbeth let himself be talked into a crime then suffered the consequences of his acquiesence. He was tragically gullible.
I believe that it is two things. Mainly, I believe that it is Macbeth's own ambition (and that of his wife) that causes the tragedy. Second, I believe that it is a lack of character on Macbeth's part. He has the ambition to be great, but lacks the moral fiber to do it in the right way. This leads to tragedy for him.
As all of you have said it is his ambition which make him a tragic hero. When his ambition is been supported by the tree witches he is on the way to his tragedy. When we have the fourth witch in the form of Lady Macbeth his tragedy is completed.