Firstly, the breakdown of the family unit and loss of the relationship triggers strong feelings of inadequacy and abandonment in the NPD/BPD parent. They cannot cope with this level of anxiety and view the other parent as a threat to their narcissistic equilibrium. The NPD/BPD parent has to find a way to restore that equilibrium and the person responsible for the disturbance, the source of their anxiety, must be destroyed.
Secondly, in this heightened psychotic state the NPD/BPD parent subconsciously references their earliest attachment model, between themselves and their own abusive parent(s), and fuses it with their current attachment model between their child and the other parent. This creates the delusion whereby the NPD/BPD parent actually believes that their ex is an abuser. Their child becomes a symbol of their own childhood abuse and they believe they have to protect their child – whether that is based on reality or not.
Any person with empathy would feel some level of sadness for the NPD/BPD parent because having a mind that operates in this way is indicative of an abusive childhood. However, the damage that a parent with NPD/BPD then goes on to inflict on their own child through AB-PA limits the amount of sympathy you feel for them. Which is just as well because they will use that sympathy as a means to manipulate you.
It should also be noted, that the “alienator” is not always a parent. Sometimes it can be a grandparent, step parent or other adult that influences the child.