Hail
The witches hail Macbeth Thane of Glamis, Thane of Cawdor and future King of Scotland. This is were Macbeths ambitions begin. If Macbeth had not heard these prophecies given by the witches he would not have gotten the desire to become king and therefore kill anyone who was in the way of that. The witches prophecies also acted as a fallback for Macbeth. He can forgive himself for killing as according to the witches he was going to become king and this was the only way.