Transcript for Former relative of 'Golden State Killer' speaks
We're turning to the golden state killer pleading guilty to killing 13 people and other crimes in the '70s and '80s. His former brother-in-law spoke exclusively to ABC news. Matt Gutman who was in the courtroom joins us from Sacramento, California, with more. Good morning to you, Matt. Reporter: Hey, good morning, Michael this. Is one of the last days that arguably one of the most sadistic serial killers in American history will wake up in this jail. Now, he pleaded guilty to that cascade of cruelty, 63 murders and rapes but for his victims yesterday may have brought justice but it wasn't closure. This morning, the sadist who terrorized California for decades forced to admit crime after crime. How do you plead? Guilty. Reporter: Joseph James Deangelo a/k/a the golden state state killer wheeled up pleading guilty to dozens of charges. The scope of Joseph Deangelo's crime spree is simply staggering, 13 known murders and almost 50 rapes between 1975 and 1986. Reporter: It took hours for the parade of prosecutors from 11 counties to read the counts against him. You attacked her. You beat her and you raped her. You murdered her in the first Reporter: And the cruelty haunting. He is the real-life version of Hannibal lecter. Reporter: Before the hearing survivors huddled together but in that ballroom his victims finally getting to stand up to him. One by one and for the Jane doe who couldn't make it Monday they all stood. During his reign of terror he taunted them with phone calls, sometimes years after the attacks. I'm gonna kill you. I'm gonna kill you. Reporter: His M.O. Was to ask for money but that's not what he was after. His abuse was all about power and control. Does it seem to you he's still trying to exert those things. I absolutely believe he's still trying to exert power and control. It's evident in there. Reporter: In 2018 he was finally captured. James huddle was Deangelo's brother-in-law. I never had a brother until he came along and became that brother. Reporter: His book "Killers keep secrets" chronicles his double life as a killer. You don't think much of and then later you find out he's this monster, evil person that is crazy, crazier than you could ever imagine. Reporter: Investigators linking DNA from a door handle and discarded tissue to decades old crime scene DNA. Does it give you any satisfaction? Huge, huge amount of satisfaction, yeah. I'm pretty overwhelmed so I don't have a whole lot of great words right now but, yes. Reporter: Now familial DNA used to crack this case is now being used by law enforcement to crack other cold cases, but it is also being used to exonerate the wrongfully convicted. All right, thank you so much for that, Matt.
This transcript has been automatically generated and may not be 100% accurate.