Causing pain can be a grey area with the determining factor being the intent behind the action. Keep in mind that any action can be used to hurt someone. Even words can cause pain if they are used in a cruel manner. This fine line can also unintentionally be crossed by not considering or other people's feelings. Sometimes pain is used to increase pleasurable sensations particularly in a sensual way.
Most people see a leather whip and they cringe at the thought of intense pain and welts.
This is usually not the case at all. When it is done right it can be just about as unpleasant a sensation as scratching an itch is. I believe that there should be a healthy balance between our dark side and our light side. I do not believe it is healthy to pretend that our darker side does not exist. Having a dark side makes us human, acting upon it in a cruel manner makes us an animal.