The sex motive is usually rape for an organized killer and sadism for a disorganized killer. They act in a series of 5 or more murders with a cooling off period between each murder. Serial killers can go on for months and years before they are usually caught. Their victims may also have the same or similar attributes in gender, age, race, general look, residence etc. Serial killers also stick by their modus operandi very closely and may change it with experience.
Most murders occur by strangulation, suffocation, stabbing etc.
Serial killers act by a sex-murder fantasy based with their control, they usually live in this dream world in their teens until they act it out for real when they get into the adult stage. As each murder occurs a serial killer may be disappointed by his murder fantasy and may act it out again to achieve it to their own satisfaction.