THE motivation for the infamous Golden State Killer to rape and murder dozens of victims may have been prompted by his hatred of his ex-fiancee, investigators say.

Joseph DeAngelo, 72, was arrested at his California home earlier this week after DNA linked to the Golden State Killer from the 70s and 80s was also linked to some of his crimes.

DeAngelo is suspected of at least 12 murders and 45 rapes over three decades.

He appeared in a US court today where he was charged with two counts of murder while shackled to a wheelchair.

He is charged with the 1978 murders of a newlywed couple who were shot to death in a Sacramento suburb while walking their dog.

DeAngelo was wearing an orange jumpsuit and made no facial expressions except to occasionally close his eyes as the judge listed the charges against him, according to the Los Angeles Times.

The alleged killer is on suicide watch in the county jail’s psychiatric ward and has been talking to himself, according to CBS News.

DeAngelo is a suspect in 10 other murders committed in central, northern and southern California between 1976 and 1986 and more than 50 rapes.

He was arrested this week after investigators, who had been searching for him for four decades, tracked him down using a genealogy database and DNA left at the crime scenes.

Paul Holes, an investigator who has spent years looking in to the case, said a critical clue to linking DeAngelo to the attacks was that the man known as the Golden State Killer yelled out “I hate you Bonnie” during one of his first attacks,The Mercury Newsreports.

It is reported that DeAngelo was once engaged to a woman named Bonnie.

“Most certainly if he’s making the statement, ‘I hate you, Bonnie,’ while he’s attacking another female, he is what we call an anger retaliatory rapist. Instead of directing his anger at what’s making him angry, he’s directing it sideways onto someone else to be able to satisfy that anger,” Mr Holes said.

“I do believe that’s what happened here. I don’t know what made him that way, but you’ve got to think Bonnie dumped him, he’s not happy about that, he still had feeling for her, who knows? But something along those lines must have happened.”

Mr Holes has helped test DNA from the various crime scenes and tie them all together since 1994.

“We always thought there was a Bonnie significant in his life, it could be a mother, a wife, a girlfriend, a childhood crush,” Mr Holes said.

Realising that DeAngelo’s ex-fiancee was named Bonnie and the DNA match helped police close in on the arrest.

The “quiet” grandpa from Citrus Heights, California, carried out his attacks in sadistic fashion. His most depraved tactic included tying up men and piling dishes on their backs while he raped their partners. He told them if the plates tumbled to the ground, he would kill them both.

Police said he entered homes up and down the coast wearing ski masks and armed with a gun and a knife. He woke his victims by shining a flashlight in their eyes and spoke through gritted teeth to disguise his voice.

Police used DNA to finally track him down and tie him to the crimes but refused to give specifics about how it was collected or matched to the suspect. They admitted that despite decades of work, Mr DeAngelo’s name had not been on authorities’ radar before last week.