It's not always the case that the "victim" has done nothing wrong. In context if you observe what's going on, the "victim" probably triggered the BPD person in some way which led to an unleashing of transference, displacement, or even projection. It may not seem like they've done anything, but in the BPD's mind, they have. And BPD's generally are highly interpersonally sensitive- actually, let me rephrase that- they are highly sensitive to rejection and abandonment, and tend to view others in terms of risks of rejection or abandonment. When you see the world with those lenses, anything from an offhand comment to displacing emotions onto the BPD because of something that happened earlier in your day... to the BPD, that equals rejection, abandonment, or the beginning of the end- the road that leads to abandonment or rejection. In other words, most people have thick skin while those with BPD often feel skinless. Anything they see, they can personalize and use it to not only process all the rage inside them from childhood, but to enact a retaliation they were unable to enact upon the significant figures who abandoned them.

Often comparisons are made to the neurodevelopment of adolescents. They, too, have difficulty accurately recognizing abandonment, rejection, and human emotion. They often take things personally and see threat where there is none. Very similar to the borderline's brain. And it's a normal human developmental course that allows the person to adapt to an unstable environment full of abandonment and rejection. One thing to recognize about personality disorders and personality formation in general is that if it exists, it is normal. A sociopathic brain develops in response to a sociopathic environment, and so on and so forth. It's a natural course of development given a certain temperament and the right environmental conditions. Look into the Romanian orphan studies, they are fascinating. This is the miracle of the human mind and body. They are adaptation machines.

In my experience, any sense of being "under attack" turns off the empathy department in the human brain. BPD's often see the world as against them and misunderstanding them. But in general, peoples' capacities for empathy fluctuate with the situation, so it's kind of limiting and black-white to say something so general as "BPD's lack empathy". They, like others, have empathy buttons that turn off when feeling like they are cornered. Your average person can see the present moment for what it is, and gauge whether or not it is worth it to retaliate or get defensive. Whereas with the BPD, the present is the past, and the past is the present. And the BPD feels the primal wound.

If you read any of Daniel Goleman's books- I'd suggest Social Intelligence- he goes into detail about the primal wound of abandonment. Evolutionarily speaking, humans were made to socialize and survive in small hunter-gatherer groups. Being left alone or "not a part of the group", in essence being rejected, activates high levels of cortisol and puts the person in survival mode because they need to be hypersensitive and vigilant. Being alone meant being prey for sabretooth tigers and the like, or being a victim to natural disaster. Fast forward to today, much of our behavior hasn't evolved. The pain of separation, rejection, abandonment, and betrayal all strike that nerve that sent the primitive human into survival mode. There's plenty of research on the effects of adoption on the child's personality development and emotional stability later in life. Borderlines and other people with severe abandonment issues have parts of their personalities arrested at the age in which they were abandoned. They learned to be highly defensive, very skeptical of others, and prone to lashing out and punishing those who abandon them. In many cases the wound is so big that they stretch small things such as microexpressions and off tones in the voice to full-on rejection and punish the person. In short, abandonment can make empathic capabilities difficult for anyone, and the borderline personality is an inevitable result of people with sensitive temperaments being abandoned during sensitive periods of neurodevelopment. Hope that adds some understanding.