HEY! Answer me this will ya?

Now, say one man has “multiple personality disorder”.
Okay, now personality #1 hates the other, (#2) and kills personality #2 would it be considered homicide?

Personally I don’t care, but if I must answer I would say yes! So he should get locked up in prison!

If the second of the last two men on Earth kills the first, does he lock himself up for murder?

Dissociative identity disorder doesn’t work that way.
You can’t kill another identity in DID.
The only way to “kill” an “alter” identity is to integrate it into the “main” identity.
If you actually kill an identity, a suicide takes place; of which there are several accounts.

This is why DID can be very dangerous when violent alter’s exist within the mind of a “main”.
However, this is rare as the vast overwhelming majority of DID cases are childhood trauma induced coping mechanisms of some fashion, and as such the alter’s are a defense mechanism against a variety of self-perceived inferiorities of the “main”.

However, putting two DID’s in unmonitored living conditions with each other is…dangerous and very nervous.
This tends to happen though, as people with DID often are outcasts even medically as their diagnosis is one of heated debate; often they are treated as full of shit and just reacting to what they learned gets them attention (this is more complicated in medical description, but it boils down to the same thing), so they often flock into community support groups for each other and as such sometimes begin to live with each other.
This, if not monitored closely by psychiatrist or, preferably, a psychologist can be a very violent instance waiting to happen as some main’s have extreme numbers of alters and some of which can be physical defense identities which can be triggered by an identity that closely resembles their childhood trauma experience in some fashion; perhaps the alter of the other person is an alter that is evoked for recreation satisfaction of their own trauma (as one such example).

But in regards to the chances of the Jekyll/Hyde or Fight Club perception of DID; this is an error.
The Jekyll/Hyde or Fight Club examples are fantasy cases of moral interest and the deaths of their alter’s is a conquest morally.
However, while it is true that (at least initially) DID individuals feel haunted by their alter’s, after therapy and guided “introduction” to “everyone” (referring to all of the alters) the establishment of the dominance of the “main” to the “alters” is addressed and is key for a goal of integration of the alter’s into the main.
Typically this can take a large amount of an individuals life, if ever; integration, depending on the number of alters.

If the introduction and main’s dominance has been established, then the alter’s and main are more less perceived as a sort of family inside of the same mind and body, by the individual, and not seen as threats to the main or confusing torments of “demons” (for lack of a better term).

For those that never reach an introduction, or establish an order of dominance of the main, it is not out of the realm to expect suicide from “insanity” (using the term loosely) of chaos and lack of control; where the killing of themselves becomes the only form of control they perceive possible over the alter’s (or “demons” if they have not yet learned of their condition or had an introduction).

The haunting, or torment, sensation is typically a result of conflicting signals in their perspective of what is not making any sense first hand in the absences of their memories, occasional seeming visual presentations of people that seem entirely real in their mind accompanied by voices at times, discovering of actions by alters counter to the ethical acceptance of the unaware main, and societal rejection and malignancy of recognition of their disorder.
They cannot possibly, in short, be experiencing what they are, yet they are, and yet they are perceiving the world to tell them they are not and are just manufacturing these things on their own even though they feel helpless to stop what now seems as self persecution (or religiously linked persecution - some see evil deities as the culprit, but this is more rare in modern times), but this all seems “insane” to their mind and senseless; keeping in mind that this mind is typically one of severe childhood trauma that does not hold self-esteem of their self high against the authority of others, typically the unfortunate cases to tend to want release from their perceived hell of being such a horrible flaw in the eyes of those around them (at least this is how it is perceived by them).
Ergo; suicide once again.

If it seems like I know fair too much on the subject; it is because my mother is a diagnosed DID with 29 different identities originally with one successful integration since the original diagnosis.
As such, I have been placed into a condition where learning of the subject matter is not only textual, but first-hand case study capable with a very long-term subject of study.

I can say assuredly that DID is real, and I can say assuredly that DID is false.
The description of what this means is an incredibly lengthy discussion in it’s own right.