The law takes into account factors that suggest a crime isn't the result of someone's free will. That means mental as well as physical.
If I have a muscular defect that causes my arm to spasm and as a result, while holding a knife, I stab you in the heart, that wouldn't be murder. No-one would suggest it was my intention to kill you. I wasn't able to control my arm.
The same applies with a mental defect. If man is unable to operate free will because of a mental illness, we don't regard that as murder. It's manslaughter (still a criminal offence).
I know it's not the sort of basic medicine you pick up from GMTV, but the medical and legal profession all agree on it.
It's not just me and his lawyer. Every politican, every newspaper, every commentator (even your crazy right-wing ones) agrees that insanity is a defence to murder, and that we should distinguish between manslaughter and murder.
The only reason you don't is because you've never bothered to think about it.