The 2003 film of this name raises interesting questions in regard to the controversial topic of multiple personalities. Without trying to decide the issue of the reality of the disorder (nevertheless, that in itself is an interesting issue!) one can extract some interesting philosophical nuggets from the logical possibility that multiple persons can inhabit a single body.
The film has us imagine that there is a way to bring personalities to interact with each other, confront each other. It also has us believe that they can, while interacting, kill each other. Obviously, this does not mean that a personality, in killing another personality, will actually cause the human being to cease biological functioning, but it does mean that the complex of memories, and personality traits that constitutes that personality will be made to irreversibly vanish. In the movie, the execution of a killer might be stayed if one personality kills what is believed to be the homicidal personality. Plot summary:
Malcolm Rivers (Pruitt Taylor Vince) is a psychotic killer awaiting execution for several vicious murders that took place at a motel. His psychiatrist, Doctor Malick (Alfred Molina), has discovered a journal belonging to Rivers that may explain why he really committed the murders. With this late evidence brought forth, a new hearing takes place on a stormy night in which Malick will try to persuade the Judge to spare Rivers. Meanwhile, a group of ten strangers, through a number of circumstances, find themselves stranded in the middle of the storm in a remote motel in the Nevada desert. With the phone lines down due to the storm, the group prepare to spend the night, taking care of those that have been injured through their arrival. However, the group quickly finds that there is an unknown murderer present, killing off each of the guests and leaving behind one of the motel's room keys to be found, starting with Room 10's key and counting down.
At the hearing, the contents of Malcolm's journal are revealed, indicating the prisoner suffers from an extreme case of Dissociative Identity Disorder, harboring ten distinct personalities. Malick is able to bring forth one of Malcolm's personalities, Ed (John Cusack), revealing that events at the motel are occurring inside Malcolm's mind, each personality being a distinct person though all sharing the same birth date. After hearing of events in the motel, Malick informs the Ed personality that he must find and eliminate the hostile personality in order to prevent Malcolm from being executed lest all the personalities be killed off. In the motel setting, as the group is dwindled down, Ed believes that the personality of Rhodes (Ray Liotta) is the murderer, and sacrifices himself to kill Rhodes, leaving only Paris (Amanda Peet) alive. When Malick demonstrates that that homicidal personality is dead, the Judge decides to place Malcolm in a mental institution under Malick's care.
As Malcolm is driven along with Malick to the institution, in Malcolm's mind, Paris has driven away from the motel to her hometown in Florida, Frostproof. As she tends an orange grove, she discovers the Room 1 motel key in the ground, and turns around to find the young Timmy standing behind her. Timmy, the true homicidal personality, had orchestrated all the deaths at the motel, and made it appear that he had been killed as well; he finishes his task by killing Paris, reciting Hughes Mearns's Antigonish. Now driven only by Timmy, Malcolm strangles Malick, and the transport truck runs off the side of the road.
As far as I know, this is not how things are with multiple personality disorder(now sometimes called Dissociative Identity Disorder DID). The personalities are only indirectly aware of each other if at all. What is more, they do not interact directly, and certainly cannot 'kill' each other. But, suppose that MPD/DID exists, and that in one such human being, the personalities can be brought to confront each other, under hypnosis by creating an environment via suggestion, as apparently is happening in this film. Suppose, also that this is done with the ultimate goal of "killing" either a single personality, from among the set, or more than one. One might imagine a therapist would aim for leaving exactly one personality intact. Suppose the chosen personality goes about killing the others. To make things hard, they are not dangerous personalities. Imagine also, as any person would, that they fear for their lives, protest, have all the anguish you or I would have at the prospect of death, as well as the full horror involved in actually being killed.
Assume all this can be done by a psychiatrist. Will that be a case that is on par with killing of normal one-personality persons? If so, who is responsible for the killings? The personality that carries out the task, or the doctor who is attempting to remedy the DID/MPD?
You might say that this line of questioning begs the question. In DID cases, reintegration occurs. So the 'killed' actually survive. What about that?
Now, if reintegration occurs, the survivor would inherit some traits and perhaps memories of the dead personalities. Is this survival of the personality, or more akin to implanting some memories into the survivor?
At first blush we might answer this way: It would seem to depend on the amount of memories in some way. If only a few are integrated, it seems implantation has happened. If all, most, or some important subset of memories and personality traits are integrated, it would seem to follow, we can claim survival.
But, is that right? Survival of person would seem to require the survival of subjectivity and autonomy, more precisely the subjectivity and autonomy of just that 'killed' person, as it had existed, and not the subjectivity/autonomy of the inheritor. Without that one key thing happening, an inheritor, even if he inherits a great many memories and personality traits, is more like a computer taking information from a thumb drive, information that came from a distinct computer somewhere else, than he is a survivor of that 'dead' person.
So, I think it is at least arguable that a psychiatrist that was able to create a scenario such as is presented in the movie Identity, would indeed be killing persons.
Now, it's a separate question as to whether this is a morally bad thing in this scenario.
A last question: is it possible to merge subjectivities, (that is subjective autonomous self conscious centers of experience), creating one from many? Is it even possible to imagine such a thing? Can you imagine yourself merging with another self?