Someone disabled to the point of having no compass for right and wrong would have what is legally known as “diminished mental capacity” and not be convicted of first degree murder.
If you genuinely hallucinate that I am an attacking Pit Bull and kill me, you are not guilty of first degree murder and I would not want you executed.
As we learn more about neuroscience, every murderer will look like they have "diminished mental capacity". This legal construct is merely a statement of our ignorance to understand what's actually going on in their brain.
It might not go that bad. But I suspect that as medical imaging hardware becomes better, we'll eventually face a crisis point: we'll be forced to re-evaluate most of what we call "character traits" and reclassify them as neurochemistry quirks. This is an important distinction, because as a society, we tend to hold people responsible for their character, but not for bugs in their brain.
We can already see some of this today. Many people that would've been shunned by society in the past as "unstable" and "crazy", can be fixed by giving them lithium. We know not to fault a person for what's just a bug in their hardware. Similarly, many people that today are called "lazy" or "annoying", and blamed for their obviously flawed character, can be instantly fixed by low doses of stimulant medication. This is something most people didn't get a memo on yet.
I find it highly likely that most homicides are also driven by fixable neurochemistry quirks, and that we'll learn to identify and fix them at some point, and we'll be appalled at the ease with which we jumped to killing people for having them.
The criminal justice system can still function without the false premise of free will and agency, and without the false premise that someone with a brain tumor magically has less agency than someone without a brain tumor but who has some more complicated and less understood problem with their brain.
It can be premised on concepts such as rehabilitation (which I believe mostly can't happen with murderers), deterrence, creating a sense of fairness in society (which builds trust), protecting people from the murderer, and so on.
If you genuinely hallucinate that I am an attacking Pit Bull and kill me, you are not guilty of first degree murder and I would not want you executed.