Many troubled people including serial killers have said "there is the bad me and the good me" and I absolutely think there is some truth to that. But let's not forget the history of MPD and the actual claim of MPD.

Robert Jay Lifton is a psychiatrist who looked at the "psychological causes and effects of wars and political violence." and he came up with the word "doubling" basically explaining how a Nazi doctor can torture and kill jews at the death camps then go home to his wife and kids and completely separate the two psychologically.

There is a great scene in the movie Frighteners where the FBI Agent thinks the guy who can see ghosts is the killer. The FBI agent asks him who he saw kill one of the victims and he replies "death" a literal cloaked figure thrusting it's hand into a person's body and crushing their hearts "until they burst".
The FBI agent tells him "This death figure is nothing more then a homicidal alter ego that satisfies your compulsion to kill! Every time you want to take somebody out death figure suddenly appears does the job for you. It's your rationalization of absolving your self of guilt."

Maybe DeAngelo really did think there was a "jerry" in him. No different then Dexter's "dark passenger". No different then Dennis Rader's frog demon.

Multiple personality disorder is not that what so ever.
Multiple personality disorder has been widely debunked since the 90s, especially after malpractice lawsuits were filed against "experts" in the field of MPD. The idea is that you are LITERALLY a totally different person. With their own memories, own life history, etc. The most famous case of MPD is of course Sybil which has been disproven and exposed as a fraudulent scam to make $$$$$$$ off of a book deal and then a movie deal. Dr.Wilbur hypothesized her patient "sybil". This exact same "therapy" has resulted in cases of "I remember being abused in the womb" "I remember being part of a satanic cult" "I remember being raped by an alien on a UFO" "I remember my own birth" honest hypnotist will tell you "people are using their imagination to go in and fill in the blanks of what they wanted to happen."

A colleague of Dr.Wilbur's would actually torture patients.
Dr.Sachs would strap patients to a hospital bed for long long periods of time, without food or water, put them on mind altering drugs and tell them they wouldn't be unstrapped UNTIL "they produced the alters", Funny enough, Dr.Wilbur told "sybil" they couldn't write a book until all her alters would "integrate" and of course like the very next day she said they had. By the way, all the "alters" sybil had were CHILDREN. Yet in the book they were made into adults or teenagers "to appeal to readers" translation: IT WAS FAKE! IT WAS A FRAUD!

The whole MPD thing was a big part of the satanic panic.
In chicago the "experts" like Dr.Sachs and others would brainwash patients to have false memories. By the way in many cases this involved entire families, children as young as 6 were being treated for "MPD" Turned out that it was a financial scam.
Very much like Salem and Matthew Hopkins. In salem not only were you killed for witch craft but your entire wealth, your land was "forfeit" and then sold at a public auction. Hopkins "witch finder general" was literally robbing each town he went to blind. He'd come into town and say "there is a witch" whip the town into a frenzy and charge the town for the expenses.

In the MPD cases, the patients had big giant health benefits plans and as one expert in insurance claims said "there was a targeting of the richer benefits plans." so it wasn't just malpractice but a total $$$$ making scam. One of the doctors asked his patient's husband if he could steal a hamburger at a family BBQ so he could have it tested for human flesh. He said he gave it to the FBI, when nothing came of it they asked him about it and he said "They got the FBI. The satanic cult controls the FBI." The doctor was a total nutjob and barely passed his examine. It's insane that he was able to look after several patients and the "star" patient [the one with the hamburger story] admitted later that when a news report was being made about them she and her doctor discussed and rehearsed her multiple personalities "to make it more believable" for the footage.

So the MPD has a long history of being a total scam and used to make a buck.

As one psychologist said "The notion of hypnotizing people. The notion of calling them by different names to label different aspects of their personality. The notion of using sodium pentathol [truth serum which has also been totally debunked] to get at repressed memories [also been debunked] that would otherwise be utterly inaccessible to their conscious mind; That has been soooo DEBUNKED, it's radioactive." Unfortunately whackjobs still use these methods to make people believe they were in a satanic cult [satanic panic never ever really went away.] raped by aliens on a UFO, were a 1600 famous person, etc. One of these conmen even admitted on camera "What makes hypnosis work is that the conscious mind doesn't know the difference between a real or imagined event."

Since doctors were being sued for malpractice [due to the satanic panic and MPD] and MPD was getting more and more looked at they decided to change it. It is now called Dissociative identity disorder.
As one expert who treats the disorder put it...when your at your grandmother's house your different then when your at your friends house or at work.