I would think if the partner were in to it they would act that way... .but I think sadism is usually found with NPD rather than BPD.
I have recently came along a site on psychopathy. Psychopathyawareness.com. My ex fit a lot of what they said there. Not all but a lot. We usually think of psychopathy in just serial killers but it is not just that. The story when he is 8 is extremely disturbing. Please don't go near that guy.