Serial killers are real-life nightmares.

He was Spain's first documented serial killer ... and the motivation behind his bloodlust defied all rational thought.

He was the sadistic killer behind one of the most chilling murder sprees in American history.

He claimed more than 24 victims ... but it was his method of murder that earned Haarmann his ghoulish moniker.

Lurking within this man of medicine was a ruthless killer who lured dozens of victims to their doom.

It takes two to make a thing go horribly wrong.

You thought you were scared of needles ... you had no idea.

May the odds be in your favor.

The crimes of these American murderers will make your skin crawl.

One woman's convalescent home came to be known as "the murder factory."

There have been several Night Stalkers over the years . . . but few were as chilling as the first.

John Christie was hanged for the murder of his wife ... but she was just one of his victims.

Four men were arrested for his crimes, but the real Monster is still at large.

By the time he was finally caught, this serial killer had claimed more than a dozen victims.

He was a serial killer, a serial liar ... or, most likely, both.

With these notorious killers, the disturbing details just keep coming ...

This killer suitor lured numerous victims into sham marriages, then murdered the women and burned their bodies in his fireplace.

She butchered three women and then cooked the remains into soap and teacakes—all in the name of breaking a family curse.

They were some of the most disturbing murderers to have stalked the earth ...

Who was the man (or woman) behind history's grisliest unsolved murder?

This lethal caregiver is suspected of killing 40 babies in medical facilities across Texas—and evaded capture thanks to a hospital cover-up.