The theme starts at the scene where Macbeth and Banquo meet the witches and they tell him the prophecy. They aren't sure if they should believe the witches but deep down they both want the prophecy to come true. (1.3.50)
The theme really starts to develop once Lady Macbeth gets the letter from Macbeth. She takes it more as an evil way then as a positive way. Macbeth is just trying to say that he may be king one day and she thinks it means that if we don't kill the king then I wont be queen and Macbeth wont be king.
Another really important part in this theme is when Macbeth is about to kill the king as sees the floating dagger. Macbeth is going crazy thinking about what he has to do even though he is a good person and doesn't want to have to kill the king.
I have supped with horrors
At the end of the play Macbeth gets killed by Macduff and ends his kingship. Since he started going crazy he started to do things as a king that people didn't like. Since Macbeth was so corrupt he started killing the people that might know that he killed the king. Which made them start wondering who really killed the king.
The tone that I am using is dark and evil. Lady Macbeth starts talking about how she needs the evil spirits to make her blood thick and "unsex me here" which means make my motherly and loving instincts disappear for this ugly and evil deed. She needs to become this "dark" because she knows that Macbeth is a good person and might chicken out and not kill the king.
I have almost forgot the taste of fears.The time has been my senses would have cooled to hear a night-shriek, and my fell of hair would at a dismal treatise rouse and stir as life were in ’t. I have supped full with horrors. Direness, familiar to my slaughterous thoughts,Cannot once start me.