@MKaze , his character progression was coward doctor then became egoist doctor and the became decent doctor, envy and amazement has very thin line of difference. why his character change so fast is because he is a young people that want to search "real me" so his character reasonably very dynamic. At first he see dr. Koto as god-like figure that he could not achieve so he couldn't do something that dr. Koto trust him.Then he had justification to hate him because the scandal and want to pushing the blame to him (and not himself)
by performing this operation and make dr. Koto looks incapable, he also want to use that to make justification that dr. Koto really was to blame for incident years ago, to justify that dr. Koto just another human and he is already surpass his former god-like figure that is dr. Koto (so he could justify that he is better than his former self) (so two reason, protect his own pride and growth justification).
After the operation he is realise that dr. Koto wasn't a God-like figure nor a mere pathetic human, but just a great doctor that he should to take example of if he want to become better than his former himself. So he make that choice so he can grow and found "greater self". Young character in story usually change and grow because they want to become a greater "self", so "why not" reason that you said doesn't click with me with this young character and the story justify that his change.
I think this is similar with Nietzsche camel-lion-baby character progression, but that is only my opinion.