good discussion. i think it serves to highlight the complexity behind the question.
can control, cant control, right from wrong aside - i think we should hold adults accountable for their actions. i think thats a healthy part of detaching. understanding is also a healthy part of detaching, but not the same as excusing or condoning.
lack of impulse control is tricky to live with and difficult for me to understand or relate to, but ive seen too many people that ive been close to lose all sense of logic, feelings become fact, and as a result they dig a deeper hole for themselves, if not for those around them. they dont so much know it at the time. they have some understanding afterward. they seemingly lack the capacity to change the pattern, and they do it again.
i dont like saying something as simple as "they lack control". they could become self aware, get into therapy, change the patterns, as others have. i do know that for them, "control" is seriously inhibited, and there are a number of psychological obstacles to get from point A to point B. its sad, and its complex to understand. on the flip side, to absolve them of any responsibility or accountability would be to enable.
survival and right from wrong: disordered thought leads to making "right" of wrong. its not the same thing as an insanity defense in a legal sense - i think what we are talking about is more complex. throw in the sorts of destructive family dynamics often seen (not always), seeds planted very early in the lives of our exes, and this too may lead to a disordered sense of right and wrong.
its not black and white. its not either or, and as

myself said, the things we are discussing are unique to the individual and on a spectrum. to say that a borderline is incapable of intentionally hurting another person - something we are all capable of and most of us have done - would be to deny them of their humanity. understanding, not excusing or condoning, informs detachment, and in my experience, helps depersonalize the wounds. it also helped me to see my own role (and by role i dont mean mistakes made or actions i regret, but the part i played) in the destructive relationship dynamics more clearly.