But these seemingly random acts of violence often follow some deliberate paths into which the innocent have unwittingly wandered.
Retired FBI agent Gregg O. McCrary spent a good part of his career climbing inside the minds of these remorseless killers to find out the method to their madness. He say's it's all about control.
"If there is any one common motive among serial killers, it's playing God, having the power over life and death of another individual," says McCrary. "It's a very intoxicating experience."
McCrary worked on just about every major serial murder case in the United States during his last 10 years with the agency.
McCrary says there is "an underlying psychopathology that drives these offenders. It could be a sexual motive...while others may be more practical homicides," such as the "Unabomber," Ted Kaczynski, who killed in a campaign against technology.
The crimes of Tommy Lynn Sells, the convicted murderer of a 13-year-old Texas girl, don't follow a single motive pattern. McCrary says some of his crimes seem to be sexually motivated while others were possibly acts of anger or a desire to eliminate witnesses.
Most serial killers are what McCrary calls "losers in life," outcasts and failures who thrive on the exhilaration gained from overpowering a victim.
Constructing a profile of an unknown assailant begins with a careful analysis of the elements, including crime scene components, autopsy data and victim selection. This is compiled and used, says McCrary, to "find the underlying pathology and learn how to use that" against the criminal.
It is not uncommon for a killer's method to evolve, says McCrary. "Crime is dynamic and (an) offender may change...and becommore comfortable over time," making it more difficult for authorities to apprehend the suspect, he says.
"They run the gamut of intelligence," adds McCrary. "The bright, adaptive ones are the ones that are most difficult to capture."
He is quick to point out that while many serial killers are mentally disturbed, few suffer from insanity. "The reality is they are not crazy, they are not insane in the terms of being legally insane," says McCrary. "They understand very well that what they are doing is wrong, but they do it anyway and they try very hard to get away with it."
By Mike Wuebben (c) MMI, Viacom Internet Services Inc., All Rights Reserved