The witches.

*Beware of Macduff
*No man of women birth can conquer Macbeth
*Macbeth will not be conquered until Birham Woods comes to Dusinane

The Weird Sisters prophecies are very powerful they are what took over Macbeths head and made him turn insane. I honestly don't think none of all this would have occurred if the witches prophecies were never to be mentioned. They simply got into Machbeths head and made him think other wise, he didn't once have a thought of being king until the three sisters came along. They just made Macbeth think that he was going to be super powerful.