The witches upset the established order by planting the seed of ambition in Macbeth's head. In their first meeting, they hail him as the future king of Scotland and as Thane of Cawdor. When he finds out that he has indeed been made Thane of Cawdor, he begins to think seriously about the possibility that he will become king:
If chance will have me king/why chance may crown me/without my stir.
Everything is as it should be before this moment. Duncan is king of Scotland and Macbeth, his worthy vassal, has just defeated Duncan's enemies. Of course, the witches could only have been predicting a future that would have come anyway. But given their obviously evil nature, it stands to reason that they knew that by doing so, they would spark trouble. This relationship between fate, the supernatural, and human agency is one of Macbeth's most important themes.
Later in the play, Hecate says that she will give Macbeth a false sense of security by conjuring the apparitions that tell him he cannot be killed by anyone of woman born, and then only when Birnam Wood marches on Dunsinane. She realizes that this will cause him to act rashly, and lead to his own destruction, and that is exactly what she plans. The sight of these apparitions:
...by the strength of their illusion
Shall draw him on to his confusion.
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.
And you all know security
Is mortals’ chiefest enemy.
Hecate does not make Macbeth's decision for him, and it seems that she cannot do that. But she gives him information that makes him act differently than he might otherwise have done. In this way, she contributes, perhaps even causes, his destruction. By meddling in Macbeth's life, the witches contribute to the bloody course of events that lead to the deaths of Duncan, Banquo, Lady Macduff and her child, Lady Macbeth, and Macbeth himself.