From the Giggling Granny to the Candy Man, history's worst serial killers are somehow even more disturbing than they sound.

The 33 Worst Serial Killers Ever To Stalk The Earth View Gallery

The stories of history's worst serial killers are more terrifying than any horror story could ever be. No make-believe monster will ever match that soul-chilling feeling that seeps in when we read a headline that says "Serial Killer on the Loose."

It's not just that serial killers are real and not at all make-believe. It's something else that truly drives the terror into our souls. It's the thought that they actually enjoy it.

Perhaps that's why they fascinate us so much. We can understand a crime of passion. The idea of bursting into a rage and losing control is only too familiar – but a serial killer is different.

A serial killer doesn't kill out of hate or anger. A serial killer moves from victim to victim, purely out of a desire to kill. For them, it's a joy, or at least a compulsion. It is — as Edmund Kemper, one of the worst serial killers of all time, put it — a thrill:

"There was actually a sexual thrill. You hear that little 'pop' and pull their heads off and hold their heads up by the hair. Whipping their heads off, their body sitting there. That'd get me off."

That's where the terror of serial killer stories truly lies. It's not just the gore and the violence of the act – it's the joy. It's the disturbing glee that a man like Edmund Kemper gets when he drives a blade through another person's neck.

For many of history's scariest serial killers, it's not just that they want to kill, it's also that they have to. It's a pulsing urge that beats inside of them. It's a demon torturing them from the inside, one that won't stop until they kill.

And then they do – and there's release. For the worst serial killers, there's no horror or fear in the act of killing. When the Son of Sam, David Berkowitz, killed his first victim, he was so filled with euphoria that he sang on his way home. He felt nothing but release.

It's hard to wrap your mind around something like that. It's hard to imagine that any human brain could go so wrong that taking another person's life could bring him great joy, or even be his sexual awakening.

But these people exist. They're real. They're not some horror conjured out of a writer's fantasy. Even the worst serial killers of all are people, made of the same flesh and blood as you and me.

We don't always even have the comfort of being able to easily distinguish what separates us from them. Most often it's likely some combination of trauma and misfiring brain cells — and that's it. A few bad experiences and a little damage to an organ in our heads and the line that separates us and the killers burns away.

And perhaps that thin line is what we most hunger to understand. That's why we're so enthralled every time we hear about Dennis Rader tying a woman up or Jeffrey Dahmer chewing on human flesh. Because we know that the real terror behind the worst serial killers is just how close they are to us.

See more in the gallery of history's worst serial killers above.

For more nightmare fuel, check out some of the creepiest quotes from serial killers or find out about the most terrifying female serial killers of all time.