Re: Guilt without knowledge
If they discovered his mental disorder, then yes, he would likely be deemed not guilty by reason of insanity, or whatever the equivalent is in the story. That doesn't mean he would escape scot-free of course, since he would be incarcerated as a mental patient eg somewhere like Broadmoor. As to interests of justice, there are two different aspects to consider. The first is the injustice to this personality who has not done the killings; the second is to the wider world who must be protected from the personality which kills and which inhabits the same body. In the view of most societies, the latter is going to win out.
NB psychosomatic killings? Or psychopathic?
EDIT: Peter sneaked in before me -- and makes more sense on the murder/manslaughter bit than me.