Would you pull the switch?
Assuming that Peter is morally competent--that he truly incurs the guilt of his deeds--then no, I would not. Peter is not ill. He does not need some sort of medical treatment. Peter is what he has chosen to become, and he should have to live with the consequences, including his own callous conscience and atrophied/ill-developed moral sensibilities. He is still free to repent of his crimes even without the magic empathy machine.
Do you think Peter would remain the same person if you did so?
I will say that I cannot see a sufficiently good reason to say that he would not be the same person. That being the case, I think it would be plausible to say that he would be the same person.
Would you still send him into prison, or do you think it would absolve him of his guilt?
Well, I wouldn't pull the switch in the first place, but if someone else were to do it, the treatment by itself wouldn't absolve his guilt (assuming he's the same person). He would still have to be truly sorry for what he's done (of course, this would be much easier with his radically altered disposition).
As for whether or not he should go to prison, I should say, first of all, that the right purpose of penal incarceration is not the absolution of guilt, but rather:
1) the protection of society from dangerous individuals,
2) the positive reformation of vicious character, and
3) the punishment of wrongdoers in proportion not exceeding the severity of their offenses because such punishment is justly due them in a
lex talionis fashion.
Of these three conditions, I would say that only 3 is, by itself, both necessary and sufficient for the just
penal incarceration of a wrongdoer. The other two are important, but they're neither necessary nor sufficient, neither by themselves, nor jointly.
So, while it wouldn't absolve his guilt, Peter may justly be incarcerated for his crimes even after his treatment. I will say, however, that it is not always and in every case meet and right that we should give a man precisely his just due. There is such a thing as mercy, and we would be very unwise to give it no heed. Therefore, since after his treatment, Peter fails to meet either of the other two conditions, I would say we should probably yield to mercy (assuming he
does express sorrow and regret over his crimes) and set him loose.
Most importantly, would you regard Peter as a moral person, even though he didn't decide to be one?
He would continue to be a guilty person
until he is sincerely sorry for what he's done, and may still justly be regarded guilty until he believably
expresses his sorrow and regret.
The real injustice, it seems to me, would be to give him a decent, moral disposition when he's done nothing to earn it. It would be an offense against his free will. We are--and ought to be--who we choose to be by the choices we make throughout our lives. No one has a right to take that away from us. In short, then, a moral disposition may be artificially imposed upon Peter, but doing so would be very unjust to him.