A spy is extremely deceptive, taking on an assumed identity.
Or a double-agent. As is a common plot in thrillers, there is suspicion that there's a mole within a spy agency, and the hero has to determine the truth. We are that hero. It's hard to detect a spy unless you do a lot of investigating, with "intelligence" (wisdom). Once you discover a spy, you want to root it out.
As Geshe-la says about self-grasping in Meaningful to Behold, in the section Delusions Bring Infinite Suffering:
If there is a group of people and one amongst them is subversive, when the other members realize who he is and what he is up to, they will waste no time in expelling him. Similarly, if we recognize the root delusion of self-grasping for that it is - our most dangerous enemy - we shall make every possible effort to eradicate it from our mind. It is not right to be complacent with such an enemy in our midst.