You said you think you need to be a sadist to be a good Dom and I don't think that is true. I have met plenty of great Doms who do not get pleasure out of causing the submissive pain. They may use it as punishment, or just be into more sensual sensory play...there are so many other options. Personally I am a masochist so I want I Dom that enjoys causing me pain (which is how I define a sadist), I don't think thats what makes those people good Doms I think it is what makes them good Doms for me.
So yeah when I think of someone as a sadist I think of someone who gets pleasure from causing pain, not as some sort of punishment and not a little spanking and pulling of the hair. A Dom (or Domme) who is actually getting off on seeing me in real pain.