Borderlines have a fundamental split between their mind and their feeling:
Borderlines start out with an attachment disorder. But on top of that something else happens. Somehow they think to understand that their parents don’t really love them. And that must mean they are awful. Hideous. Disgusting.
They feel it would be better if their true self no longer existed. And that is when they split. They split into two halves: Mind and Feeling. The Feeling hides inside, deep deep deep inside. And it hides because that is the part that is hideous.
The Mind remains behind, alone, in charge of dealing with life, but without his Feeling.
This happens at a very very young age: before the age of 2 1/2. The Mind becomes aware after that and it becomes aware in a status quo. He doesn’t know the Feeling has split off, he doesn’t know he is “just” the Mind, he thinks he is a whole person, and so does everyone else.
The splitting that a borderline does with the black and white is both a clue to the age at which the split between Mind and Feeling took place and a result of the split.
Splitting is a very normal mechanism for young children. They have no sense of time, they only know the NOW and all that they know is that the person with them is leaving them. He has no concept of “coming back” so he despairs: I am being abandoned, and you are awful for abandoning me.
Around the age of 2 1/2 children start to get a concept of time, of “coming back,” of love that stays even when the person giving love is absent. That is when splitting starts to go away.
The very fact that borderlines still see black and white is a clue to when the split between Mind and Feeling happened: before 2 1/2 years of age. Because if it happened AFTER that moment, they would have unlearned splitting.
Now, what happens when the borderline grows older is that his inner Feeling never grows older. It never gets a sense of time, it never acquires the feeling of “object constancy:” that love will stay even when the person leaves.
The result is that the borderline is never safe and secure. But because of the split between Mind and Feeling, the borderline doesn’t know this. It’s the Feeling who feels this, but because there is no longer a connection between Mind and Feeling, the Mind doesn’t know. The Mind tries his best, but doesn’t know what’s ailing him, and nobody else knows either. Everyone, the borderline included, thinks that he is a whole person.
And at the moment that the Feeling, deep deep inside, feels threatened, feels that the other is abandoning him, or that the other is on the verge of discovering the Feeling (both are equally horrifying and must be prevented at all cost), the Feeling takes over. But since the Feeling is only 2 1/2 years old at the most, he only has one way of responding: with rage.
And the Mind is as much surprised as the person at whom the rage is aimed.
<<Laura Hurt's answer to Do people with BPD split on themselves?>>
The borderline can hardly be held responsible for actions that follow from this split. He doesn’t know that he is missing part of himself, he doesn’t know his Feeling is hiding deep inside, and he is not responsible for the Feeling lashing out.
Anything else is personal, and the borderline is as responsible for that as any other person. Decides to stalk and harass an ex? Not borderline, just a plain asshole. Decides to leave current partner after cheating with someone else? Not borderline, just a plain asshole.
So it really depends on what exactly is the issue. I think that too often other people conflate the two things, and that some borderlines hide behind it to escape blame (and that again is personal, not due to borderline).
Quora - Laura Hurt