Hi Worried, are you sure you mean borderline personality disorder, and not bipolar?
A personality disorder is not technically a disease or condition, and certainly not one caused by MS. Although it is “diagnosable”, it’s still controversial in the sense it’s arguably part of the person’s actual personality, and not something that has “gone wrong” with them (i.e. not like PTSD).
The very existence of borderline personality disorder is controversial, with not all psychologists accepting it. I have a friend - actually a penpal (we’ve never met) who is so diagnosed. I suppose you could argue it’s hard to tell just from correspondence, but we’ve been exchanging e-mails some 15 years, so I feel I know her. I do not recognize the anger or nastiness you speak of, but she is prone to extreme insecurity - for example, despite the long-lived nature of our correspondence, she is sometimes seized with the conviction I would not want to write to her any more.
“But I’ve been doing it 15 years! What’s brought this on?”
“Dunno, sorry, nothing… I think I might be being silly.”
It’s even more apparent in her intimate relationships - she fears abandonment by people close to her, and tends to form volatile and sometimes inappropriate (i.e. illicit) relationships. The paradoxes around BPD are sometimes summed up by the phase: “I hate you, don’t leave me!”
My friend is impulsive but fragile. She sometimes has thoughts of self-harm, but not anything she has put into action. They are more like morbid thoughts she might be seized with a sudden urge than actual intentions.
None of this sounds like what you are describing, and anyway, MS would not cause a personality disorder. Someone either has one or they don’t. If they have one, it’s likely their personality was either always like it, or at least from a very early developmental stage. It’s not “brought on” by disease or trauma - or if it is, it would have to be extremely early trauma, at a time when the personality was still forming. Not adult onset MS. Sorry, doesn’t fit. MS doesn’t cause PDs.