Bi polar and BPD are different but often have over lapping symptoms. For example I have bipolar disorder II (no full manias, we don't fully loose touch with reality) which was rapid cycling so I would have a full high and low within days. In addition to these symptoms I also presenting symptoms of BPD but I was 'down one symptom' for a full diagnosis as they were attributed to the bipolar. Weird and complicated.
Anyway, often the behaviours can look the same or similar. My understanding from my own diagnosis is that bipolar is often more of a chemical issue that can be triggered by environmental factors where as BPD is more likely to be a direct result of environmental trauma.
I'm sure there are mh professionals around that can tell me I may have got that entirely wrong, that's just my understanding from talking to my mh professionals, however, that ,ugh the be just how it is in my case iyswim.
But yes, there are arses in all walks or life, sometimes they have a mh issue, that doesn't always cause them to be an arse though, sometimes they're just that!