The three witches in Macbeth are the key to the stories turn of events. They control the lives and destiny of every single character in the play. Everyone's lives in this play wouldn't have been as drastically changed if it wasn't for the witches and their deceptive fortune telling.
Actually, it should be called, "misfortune telling". Ever since Macbeth returned from his battle, he was intrigued by the witche's call that he would soon be King. "All hail, Macbeth, that shalt be King hereafter." (I.iii.50). The greed and pre-planning of his future rule was already taking place in Macbeth's mind. Macbeth might have even been able to accept his rule more gracefully if the witches wouldn't have been so vague in their predictions. Macbeth had to act on a whim, guessing what he had to do next, when he receives his title. Murder and lying clearly weren't the right solutions to Macbeths future successes.
Towards the end, after several "misfortune telling" from the hags, the gave Macbeth another hint, or tip. The second apparition said, "Be bloody, bold, and resolute! Laugh to scorn the power of man, for none of woman born shall harm Macbeth." (IV.i.80-82). This only added to Macbeth's excessive greed and willingness to stop anyone that's poses a threat to his advance in the hierarchy. The apparition also ceases Macbeth's ability to think things through. He did not realize that there are some people that aren't born to conventional methods. .
This sure did kick the last leg out from under Macbeth's not so sturdy table. Macbeth's death shows that the witches were some bad dudes. Should they have been trusted? I guess so, but Macbeth's actions were clearly traveling faster than his mind could think. Bad, bad witches they were.