For Banquo’s issue have I filed my mind;
For them the gracious Duncan have I murder’d;
Put rancours in the vessel of my peace
Only for them; and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!
– William Shakespeare
Macbeth, Act 3, Scene 1. An upset Macbeth realizes that he has sold his soul – “eternal jewel” (metaphor) – so that Banquo’s descendants will be kings. For that he has killed Duncan and destroyed his peace of mind. That is where his ambition has brought him to.