This taken from the MIND website:
If you are charged with murder and can show that you had an 'abnormality of the mind' when you committed the crime, the court will convict you of manslaughter instead. This could mean less punishment. This is called 'diminished responsibility'.
Criminal charges - during your trial - Mind
I think you'll find Schizophrenia is an "abnormality of the mind".
"Schizophrenia is a serious mental illness that affects how a person thinks, feels, and behaves. People with schizophrenia may seem like they have lost touch with reality".
In your first post you state that it was "pretty much proved in court". Is that "beyond all reasonable doubt" or just what you perceive?
I'm not condoning or offering to excuse what this man did but your post is a bit narrow minded and IMO, poor and simplistic.