Can a person with mental illness be charged with man slaughter?

they would probably be found guilty due to diminished responsibility thus getting a much lighter sentence.

you have to prove he knew what he was doing and mental illness played no part in this murder.

Generally in law mental illness or disorder is not a total defence to a criminal charge, but instead is classed as a 'mitigating factor' (i.e. something which lessens the seriousness of the charge) unless it can be reasonably proven that the mental illness or disorder is so bad as to render the person incapable of distinguishing between right/wrong actions and from acting accordingly, or that the manifestation of that mental disorder is such that it caused the person to act as they did.

To answer the question with regard to manslaughter, there is no current legal reason for the non-prosecution of someone with a mental disorder for manslaughter.

First, it's important to remember that anyone can be charged with anything. Whether or not they can be convicted is another matter.

Mental illness is not itself a defence to a criminal charge. It has to be a mental illness that stopped the perpetrator from knowing what they were doing. Lennie's mental deficiency wouldn't qualify him for that.

However, a judge may still consider his mental disability in sentencing.

Yes - and I wholeheartedly agree with Steven's answer too.

Mental illness is not a get out of jail free card. Check out the book "Helter Skelter" by Vincent Bugliosi.