WARNING: GRAPHIC CONTENT - Patrick Kearney was bullied a child in school, but as an adult he took it out on the world by slaying dozens of victims over two decades

A serial killer with a "genius IQ" first showed signs of a disturbing personality by slaughtering pigs when he was just 13.

Patrick Kearney was convicted of murdering 21 people - but police believe he could be responsible for many more. Born in 1939, as a teenager he took a twisted enjoyment out of shooting swine behind the ear, something his dad thought might toughen him up.

It wasn't too long before he'd mastered the technique and relished in it so much that he wallowed in dead animals' blood and innards. At school in Los Angeles, California, the youngster was thin and frail and suffered from bullying.

He would soon begin dreaming of murdering those who had degraded him. He eventually finished his education and enlisted in the Air Force in Texas, where he met David Hill.

His new friend was married, but that didn't stop the pair embarking on a romantic relationship and, once Kearney received an honorable discharge, they relocated to California as partners.

But it didn't work out and Hill returned to his family in 1962, the same year that Kearney's thirst for human blood became a reality.

In spring of that year, he offered a lift to a 19-year-old hitchhiker on his motorbike. He took the unsuspecting man to a secluded location and shot him behind the ear, precisely like the pigs he had killed as a teenager, before sexually abusing his victim's body.

But he feared the victim's cousin may have witnessed his victim being picked up - so he killed him in exactly the same way. Another murder followed soon after - yet another teenage youngster picked up from the streets.

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Kearney reunited with Hill in 1963 and it wasn't until 1968 that he killed again, this time it was a friend of his lover's. He crept into the man's quarters and fired a pistol shot between his eyes.

He hauled the corpse into a bath, sexually abused it and then cut it apart with a blade. He even extracted the bullet from the man's head before interring the remains behind the garage and composedly returning to California.

When Hill left again in 1973, the killing spree resumed. At least 20 more people - including hitchhikers, sex workers, men in pubs and an eight-year-old boy - were slaughtered.

The vast majority Kearney picked up were shot in the head, with many carefully selected victims resembling those who bullied him in school.

A method Kearney used was to drive with his left hand, then shoot the victim in the head with his right hand. Kearney would then position the corpse upright in the passenger seat to create the illusion he had company, before driving to a quiet spot and abusing the corpses.

He would then chop them up and stuff the remains in bin bags before scattering them across various sites, often highways, which earned him the nicknames 'The Trash Bag Killer' and 'The Freeway Killer'.

After 15 years of murdering innocent people, the police finally caught up with Kearney in 1977 when an investigation into the death of a 17-year-old called John LaMay led back to him. After his arrest, the full horror came out as he confessed to a staggering 35 killings.

A psychiatrist who examined Kearney reportedly claimed he had an IQ of 180, way beyond the level of "genius". For comparison, Albert Einstein's IQ was only said to be around 160.

To avoid the death penalty, Kearney pleaded guilty and was convicted of 21 deaths in total - police didn't have enough evidence for any others. He was sentenced to life behind bars, where he remains today in California's Mule Creek State Prison.