Serial killers who claimed the most lives

Crime fills the news day in and day out. But every now and then, a particularly brutal criminal surfaces, making us question everything and everyone around us. A serial killer is defined as an individual who has killed more than three victims on two or more separate occasions. Who are the worst serial killers to have walked the face of the planet in modern history?Click through the gallery to find out who the world's most prolific killers are.