Black and white thinking is based on absolute extremes. Good vs Bad/Evil.
When a person is viewed as "good", they are placed on a pedestal and can do no wrong and we adore them.
When that same person is viewed as "bad" (for Borderline PD this can happen rapidly) then we strip them of value and they become objects of scorn, ridicule, debasement, revenge.
Trouble is, no one remains on a pedestal forever. Because we are all human. But when placed on that pedestal, our expectations are unrealistically perfectionistic and the disappointment of the impossible unfulfilled expecatations is real painful, and can feel like a betrayal.
It is even more complex that all of the above when you explore it more.