This is known as splitting. The good person is idealized and the bad person is devalued, there is no in between. It is the black and the white, there is no grey area in the world of the unrecovered Borderline. The depth to which most Borderlines feel their pain is for the most part not understandable to non-borderline individuals. This deep intrapsychic pain is often the pain of a traumatic childhood.
Borderlines live in constant fear, terror of having to deal with real or often imagined abandonment.
Attachments and bonds are very difficult for borderlines to develop because there are many control and trust issues with which they do not cope well. They have a strong need to protect themselves from anymore pain which sees most borderlines basically being incapable of dealing with their own vulnerablities or the vulnerabilities and emotions of others. Borderline individuals may not seem it to the outside world around them but they are very sensitive people in a great deal of pain. The very unfortuate reality of this personality disorder is that when they need and what they need to the most Borderlines often are compelled by impulse to push away, to sabotoge in order to protect themselves from the agony increasing that is ever present inside. Borderlines, not unlike anyone often project, to a greater degree, grant it than the average.