Once a respected archaeologist and the leading expert on all things simian, Lord Montgomery Fiske is an example of the consequences of obsession.
Monkey Fist considers Ron Stoppable as his personal nemesis, despite Ron's role as Kim's sidekick. His subsequent plots have all revolved around mystical monkey power in one way or the other, which indicates that he has trouble letting go of his obsessions.
In his final appearance, Monkey Fist was turned into stone for following the path of the Yono. His lust for power had consumed him by that point, and he was willing to do anything and everything to ensure that he would be the monkey master, even if that meant giving up his soul.