Although it would be unwise to negate Lady Macbeth's role in the matter, the witches in Macbeth can be said to be responsible as their predictions come true. In Act I when the witches chant "Fair is foul and foul is fair," they set the tone for the play--"nothing is but what is not" or things are not as they appear to be. This also begins one of the themes of the play--appearance vs. reality. The witches also chant "something wicked this way comes" when Macbeth first enters the play. At this point in the play, Macbeth is not wicked, but the witches plant thoughts of his rise to power when they predict that he will become king--which of course comes to pass when Macbeth assassinates King Duncan. When Macbeth calls on the witches to show him the future, they predict Banquo's son becoming king, the attack of Macduff on Macbeth's castle and Malcolm becoming King. Because the witches are used by Shakespeare to predict the future and introduce the concept and poetic device of equivocation, the witches can be deemed as responsible.
It can also be argued that the witches are not responsible for Macbeth's failures and the murders he commits because he makes those decisions on his own and in order to please his wife. Also, the witches are merely messengers of future events; they don't create the future. In this way, the witches could be likened to the concept of Fate which was a popular notion in classical Greek and Roman literature as well as with Shakespeare's plays. The witches could be considered an evil side of Fate and complement the play because of the play's violent, murderous, and bloody premise. However, Fate's nature is to declare a person's path and end--that's all. The problem comes when humans try to stop themselves from traveling that path, or in Macbeth's case, when humans try to force the path and its end to happen sooner or try to change the end. The message to Macbeth was that he would be king, but that Banquo's sons would reign afterwards.
"All hail, Macbeth, that shalt be king hereafter!" (I.iii.53).
"Thou shalt get kings, though thou be none./ So all hail Macbeth and Banquo!" (I.iii.70-71).
These two quotes are all that Banquo and Macbeth are given as far as the message is concerned. Had Macbeth let all alone, the end still would have been fulfilled but maybe the means by which is was achieved could have been less bloody. And according to the rules of tragedy, the hero's fatal flaw must bring about his end, not the voice of Fate. It is Macbeth's poor choices driven by pride, ambition, and desires to prove himself to his wife that condemn him.