How does this prove that one possible theme in William Shakespeare's Macbeth is "Uncontrolled ambition can lead to negative consequences" because it shows that....

I'm not sure what scene you are referring to, but the theme of uncontrolled ambition and its consequences in distinctly apparent in the scene in which Macbeth decides to kill Banquo. The prophecy of the witches quickly becomes ingrained in Macbeth, and Banquo, as a part of the prophecy, is a threat.... a "rancour" within Macbeth, and a malignancy he needs to remove.