If a person suffering from dissociative identity disorder ( multiple personality disorder) kills themselves then they are a serial killer. amirite?

I think you mean mass murderer

If they threaten to commit suicide is it a hostage situation

A serial killer kills multiple times.

Killing multiple people one time doesn't make you a serial killer.

It could also be argued that if the intent was to kill themselves and they did not intend the other personalities to die (if they were even aware of them) then it's possible they are only really guilty of manslaughter.

Masturbation might be sexual assault