Concentrating on just the final paragraph first, because it provokes the most interesting answer IMO.

Imagine a heinous murder in which the killer did it “just for the fun of it”. Yet upon psychiatric and medical examination he is found to have a tumor the size of a golf ball in the medial prefrontal cortex of his brain (this area is responsible for emotional control and behavioral impulse). It would be fairly easy to surmise that he was not in any real sense responsible for his actions in carrying out the murder.

Really, why?

He was not in his right mind.

Not quite. He was his mind; where he refers to the man with a tumor. He was not in tumor-free-man's right mind. So we punish man-with-tumor, not man-without-tumor, as they are clearly very different people.

We would not prescribe the same punishment for him as we would a perfectly healthy individual.

Depends which state you live in. By removing the tumor, we are essentially killing man-with-tumor. Replacing him with man-without-tumor, a completely different person. If you live somewhere with the death penalty, that is in fact the punishment you would give a healthy individual.

Would it be moral to deny this man surgery as a ”punishment” for his crime?

In what way would that be a punishment for man-with-tumor, the entity that commited the crime? Man-without-tumor would be punished by that, due to continued non-existance, but man-with-tumor would not.