The witches are the catalyst for all events in Macbeth, and without them, neither Duncan nor Banquo would have died
However, these are the only two titles that Macbeth came about honorably. As the play progresses, Macbeth's scheming wife convinces him that the meaning of the witches' prophecy was that Macbeth would kill King Duncan and take his throne out from under him and his sons. Lady Macbeth's interpretation of the prophecy leads to her starting to convince Macbeth to take action against Duncan and teling him, "To beguile the time,/ Look like the time. Bear welcome in your eye,/ Your hand, your tongue. Look like th' innocent flower,/ But be the serpent under 't" (1.6.75-78).
Macbeth brushes her off, telling her that they'll talk more about it later. After Lady Macbeth graciously recieves Duncan and company, Macbeth says, "If it were when 'tis done, then 'twere well/ It were done quickly." (1.7.1-2) Macbeth is saying that if everything was over and done with after Duncan's murder, then it would best if Duncan was killed quickly. Lady Macbeth enters, and the two have a heated conversation about their assassination plan, though Macbeth is still reluctant to take action.
HIs wife insults his manhood and courage again, which upsets Macbeth, and he decides that he will follow his wife's will and murder Dunacn. Macbeth goes on to hallucinate a dagger out of guilt, and then proceeds to Duncan's chamber, where he murders the king and his two guards. Duncan's two sons flee Scotalnd to preserve their lives, and the eldest goes to England to try and get help from the king. After this, Macbeth takes the throne, but has gone power-crazy, and sends for two murderers to kill Banuqo and his son so that they will not take his crown from him.
The son escapes, but Banquo is not so lucky. A suspicious Scottish noble goes to visit Duncan's eldest and comes back with Malcom and an entire army to overthrow Macbeth. Macbeth, at this point, is completely mad and reluctantly enters the battle. Macduff, the suspicious noble, is the only one capable of killing Macbeth, and fights the corrupt king. Macbeth loses, and is beheaded, thus ending his reign as well as his suffering.