Some people who have sadistic sexual fantasies mean no real harm to anyone. In BDSM a "top" or dominant person will bind his or her partner or "punish" them in different ways, but what they do will not go beyond limits which the partner has agreed on. On the other hand, a small minority are severely perverted or mentally ill, and wish to act out their sadistic fantasies for real. Such people are rare, but they can be very dangerous. Many serial killers and psychopaths have been sexual sadists.