BPD is thought to be genetic. A previous of mine partner was BPD though I didn't realize it at the time.
He had not had a good childhood, from what he told me I suspect his father was BPD and his mother NPD.
His childhood was OK as much of the time he was looked after by a much loved aunt.

He says he was happy and always smiling till he was about 30.
What happened at 30?
His mother and father emigrated to Australia to be soon joined by his eldest brother (the favorite child)

This is when he went into freefall, a huge sense of abandonment engulfed him and mental health problems came to the surface.

Once he'd caught her making out with a guy in their kitchen, he was horrified, and refused to have anything to do with this family friend. After a few weeks however his mother ordered him to be friendly and talk to this person, he was expected to act like nothing had happened.

His mother was not a good mother, but he loved her. (dunno why, but that's just me sorry)

I wonder, if he'd been adopted as a young baby how would he have been then?

He would have still been BPD, but I think with good 'normal' loving parents his PD would had been minimized and mostly controllable.

Nature loads the gun, and nurture pulls the trigger.