If the Weird Sisters truly know the future and are not simply manipulating Macbeth into thinking that they do, then everything that happens in the play is the result of fate, or destiny, and not effects of Macbeth's corruption. In other words, if the future is knowable, then it must be fated, and if it is fated, then no one can be blamed for their role in bringing it about; Macbeth is exonerated from responsibility for his deeds. For example, the witches tell Banquo, "Thou shalt get kings, though thou be none" (1.3.68). In other words, Banquo's line of descendants will produce kings. This is true. The king on the throne when Macbeth is first performed is King James I (James VI of Scotland), and he can trace his lineage back to Banquo.
Further, when the Weird Sisters call up the apparitions in Act 4, scene 1, the first is an "armed head" (a disembodied head, wearing a helmet) which says that Macbeth must "Beware Macduff" (4.1.73). It turns out that Macbeth should fear Macduff because it will be Macduff who kills him by cutting off his head after Macbeth has donned his armor. Therefore, the apparition -- created and conjured by the Weird Sisters -- not only knows the future but takes the shape of something to occur in the future as well. Likewise, the second apparition appears as a "bloody child" who says, "Laugh to scorn / The power of man, for none of woman born / Shall harm Macbeth" (4.1.81-83). Macduff was not technically born, as he tells us later, he was from his "mother's womb / Untimely ripped," i.e. he was born via Caesarean section (5.8.15-16). So, the apparition appears as a bloody child because a child born by C-section would be bloody since the mother's been cut, and it knows the future: that Macbeth will be killed by someone not "born." Finally, the last apparition, "a child crowned, with a tree in his hand," says that "Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (4.1.96-98). The child's appearance gives a clue as to how this will happen: a rightful king (Malcolm, child of Duncan) will march on Dunsinane with his army, each soldier holding up a bough of a tree in order to shield his army's numbers from Macbeth's lookouts. Again, the apparition (produced by the witches) knows the future.
Ultimately, then, Macduff isn't in control of his fate: he is destined to kill Macbeth. Malcolm isn't in control of his fate: he is destined to lead an army against Macbeth. This means that Macbeth cannot be in control of his fate either: he must perform the actions that lead him into this place at this time, and thus we cannot fault him for killing Duncan because that action was only the one to set this destiny into motion. It was fated too. If we accept that the Weird Sisters know the future, then that means the future is knowable, and if the future is knowable then we do not possess the free will to change it; we are only pawns of fate and thus not responsible for our actions.