I don't think you could punish the killer. I would want them to be treated and hopefully their minds could be reconciled. They have to work together and can't be of significantly split mind or else be useless much like a single person who is internally conflicted. Considering their permanent proximity to each other, I would expect them to be very heavily influenced by each other, and in that sense, I tend to want to treat them as a single person.
In some sense, I think the dilemma is a lot like what we really face all the time with people, and with ourselves, insofar as we feel inclined to think of a person as being taken hostage by guilty, hurtful, destructive thinking. It is generous and merciful for us to have that viewpoint ... truthfully I kind of like it, and would rather see people that way. I would rather see myself that way. I think of the irony of people saying things like "now you show your true colors!" when they do something evil, because the opposite may be the truth. People say when they do something they regret, "I was not myself". Their recognition of that tends to more easily inspire our forgiveness if we felt hurt by them, because our forgiveness amounts to the same thought pretty straightforwardly: who they are = that which is innocent.
