There is a critical distinction to be made between mental defects that compromise one's ability to think rationally vs defects that simply overpower one's motiv

There is a critical distinction to be made between mental defects that compromise one's ability to think rationally vs defects that simply overpower one's motivation to act in a rational way. If you can think straight and accurately perceive your environment, you are responsible for your actions. As a rational being, your plight is to see beyond all those powerful monkey emotions and do what you know is right. Welcome to humanity. If you really are out of your mind and your brain is just playing tricks on you, you're off the hook morally, though if it can't be fixed then ultimately you can't play in the human sandbox, sorry. Since none of us are even close to perfect rational-observation machines, we have to cut ourselves some slack. How much slack is an incredibly difficult decision, but that's the decision at the core of our practical ethics. Without making some distinction between motives and choices, our ethics are pretty meaningless.