Serial killers are defined by the FBI as individuals who murder three or more people in separate events over time, often driven by psychological motives. "Dangerous" here refers to those with the highest confirmed victim counts, the brutality of their methods, and the societal terror they inflicted, based on historical records and law enforcement data. Below is a table highlighting 10 of the most notorious, ranked primarily by confirmed or estimated victims, drawing from comprehensive lists and investigations. These figures are substantiated by convictions, confessions, and forensic evidence, though exact numbers can vary due to unsolved cases.