The psychopath understands their actions are illegal and not acceptable to society. They may not “feel” it (as in why society has come to this conclusion) or care or they feed their own internal needs/impulse - all of which gives society the “right and responsibility” to remove them from society (which method, psychiatric facility or prison is subjective).
A drunk driver doesn’t have a legal “excuse” when he is so intoxicated he didn’t “know” …a person may have an undiagnosed mental illness and still be legally responsible and convicted because there is an understanding of societal law.
Edited to add: what if I bought into the simulation idea to such a degree I convinced myself that other people are just “background characters that are programmed” - does that mental thought pattern absolve me legally from killing -
What about the “devil made me do it” or “I was on a mission from God”… (re:killing/murder)