“Golden State Killer” Joseph DeAngelo reportedly tried to blame his gruesome murders on having a split personality named Jerry, who “made” him commit crimes against his will … the problem is that actually hurts your case in California.

This is according to a new book — “The People vs. the Golden State Killer” — by Sacramento County District Attorney Thien Ho.

Ho writes DeAngelo muttered to himself in interrogations, saying things like, “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’ve destroyed all their lives … I raped. So now I gotta pay the price.”

Ho, who was the lead prosecutor in the case, explained … “He tried to establish this split personality defense, but he did it in such an amateurish way that he defeated his insanity defense because insanity in the State of California, you only get it if you don’t understand the nature of the wrongfulness of your crimes.”

Serial killer David Berkowitz — AKA the “Son of Sam” — tried a similarly ham-fisted move when he initially blamed his murders on his neighbor’s dog, Harvey. Berkowitz claimed there was a demon speaking through the Labrador who commanded him to kill. He later admitted he had made the whole thing up, hoping it would help him score an insanity defense.

DeAngelo, 80, was taken into custody April 24, 2018 for killings and rapes committed in the 1970s and ’80s. He’s now incarcerated at California State Prison – Corcoran, serving multiple consecutive life sentences without the possibility of parole.