Free will is the notion that what we do comes from us and is not down to some programming or factor that makes us choose what we choose. It is the view that when we choose we really choose and it is not some kind of illusion. Some believers in free will hold that not all of us have complete free will though some do. Some believe that all of us have enough free will to be reasonably accountable even if not fully accountable for what we do.
Belief in free will is not worth it if it demands that we treat insane people as guilty of the evil they may do.
Let us assume free will is a reality. The insane person may have hallucinations but that does not mean that what he or she does in terms of evil is not done of their own free will. They still have intact power to choose. Intention and insanity are two separate things.

If we regard insane people as not guilty or responsible for the evil or harm they do then what about everybody else? Insane people think they are sane. You cannot fully know then if you are really responsible for the evil you do.

If you believe in free will, then you have to treat sane and insane alike. This stance is outrageous and evil. Belief in free will should lead us to do grave injustice to the insane. If we don’t do it, then we are still cuddling up to a principle that says we should and demands that we do it.
If you think you have free will, you will end up feeling that you own the mental illness you have and that you own the shame. It makes your intuition tell you that if you were the person next door you would not be so ill. Free will is the reason there is a stigma against those who suffer from mental illnesses and who are psychotic. No such stigma exists for any other kind of illness.
Some argue that if you are a really good person and suffer brain damage and turn into a serial killer that you are not responsible. They say you are doing what you would not do if your brain were okay so you are not accountable. But others contest that. They say, "The question is what is a normal brain when we are are all so different from everybody else. And what if you are born with genes that make you a savage? Being different or damaged does not mean you are no longer you when you do what you do."  What if your brain has a way of making you stay you even when damaged at least in a certain way?

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