Hmm, well the BDSM side of things is something I became intrigued with. Not on a personal level but a woman who I ended up helping to run a web forum with used to be a dominatrix and the concept was so far away from who I am I found her explanation very interesting.
She was very open but equally her life is chaotic and she lurches from one flawed relationship to another, so I don't think she is a great example of a balanced person.
According to her a good sadist has 2 sides, 1st to inflict pain and humiliation on the sub in a controlled manner ensuring that they only inflict as much physical pain an mental humiliation as the sub can handle but then they also have the responsibility to reassure and care for the sub afterwards to ensure they are protected mentally.
The sub however has the ultimate control, they have the safe word, they should have ultimate trust in their dom and they facilitate the domination.
If the sadist is not working with a sub with a level of trust and the sub is not willingly submitting to the domination then that is abuse.
So what should make a sadist is a willing sub and a predisposition to enjoy inflicting pain and humiliation.
I have no idea if that helps or not!