Joseph DeAngelo has pleaded guilty to charges relating to the so-called "Golden State Killer" spree of murders and rapes in the 1970s and 1980s.
In a courtroom in Sacramento, California, prosecutor Thien Ho said that DeAngelo had been left alone in a police interrogation room in April 2018 when he started talking to himself.
According to Mr Ho, the former police officer said: "I did all that. I didn't have the strength to push him out. He made me. He went with me. It was like in my head, I mean, he's a part of me. I didn't want to do those things. I pushed Jerry out and had a happy life. I did all those things. I destroyed all their lives. So now I've got to pay the price."
DeAngelo, 74, was pleading guilty to 13 murders which will see him jailed for life without parole but means he will avoid the death penalty.
Referring to the murders and more than 50 rapes, Mr Ho said: "The scope of Joseph DeAngelo's crimes is simply staggering. Each time he escaped, slipping away silently into the night."
Looking frail and dressed in an orange jumpsuit, DeAngelo sat in the courtroom with a face visor to guard against the spread of coronavirus.
Before the hearing Jennifer Carole – whose mother Charlene Smith was raped and murdered, while her husband Lyman Smith was also murdered – told the Associated Press: "I've been on pins and needles because I just don't like that our lives are tied to him, again."
Two survivors, Gay and Bob Hardwick, told the AP that they would have liked DeAngelo to have been executed but thought that was never realistic, given his age and California's moratorium on the death penalty.
Ms Hardwick said: "He certainly does deserve to die, in my view, so I am seeing that he is trading the death penalty for death in prison.
"It will be good to put the thing to rest. I think he will never serve the sentence that we have served — we've served the sentence for 42 years."
DeAngelo was caught after authorities used DNA to track him through a genealogy website.
The Associated Press contributed to this report
