Macbeth is a loyal subject of the king, awarded for his bravery in battle. Once he meets the three witches, and they provide him with the prophecy, he surrenders his ethics, his morality, his honor to ambition, to the pursuit of power.
It is as if the witches awake in him a deeply held secret desire that only they can stir. Being women, they are able to hold sway over him, he is easily influenced and the witches use him as a play thing for their enjoyment.
The same can be said of how he is influenced by his wife. When he tells Lady Macbeth about the witches prophecy and that King Duncan is coming to visit, she begs, pleads and threatens him into believing that he must kill the king. He doesn't want to do it, but she is very convincing, linking his ability to be a man with his ability to kill the king. Linking his love for her with his desire to kill the king.
Lady Macbeth twists Macbeth around her finger, between this woman and the three witches, Macbeth is transformed into believing that he must grasp at this grand opportunity, after all, it is destined, fated to be so.
The witches and Lady Macbeth are partly responsible for the murders that take place in the play.
One could try and make the argument of the strong female types in MacBeth and comment that the female characters in MacBeth have a devastating effect on the males in the play. It is significant that the witches are women because traditionally, women were witches and most witches were reputed to have strange and arcane powers and had the ability to manipulate both elements of nature and the respective genders. Lady MacBeth and Lady MacDuff are other characters in the play that could be interpreted as strong and "effective" characters.