Macbeth cut open Macdonawld because there was a war between Scotland and Irish.
The witches told Macbeth that he was going to be the 'Thane of Cawdor'. Later on, the King of Scotland.
Because of Macbeth's bravery King Duncan named him the 'Thane Of Cawdor' while thinking later on he would be named as King of Scotland but King Duncan named his son Malcom as the next King.
Lady Macbeth reads Macbeth's letter announcing that he is now the 'Thane of Cawdor' but she expected something more bigger and she decided to take control over the situation and thinks about killing King Duncan.
Lady Macbeth influence Macbeth to kill King Duncan
Lady Macbeth and Macbeth are really excited because they believe that they are finally going to have the power they never had before.