The law said the defence can be raised when there is 'any evidence, which if believed, might create a reasonable doubt'.
Expert testimony isn't required, the evidence can even come from the circumstances of the crime, the defendants behaviour, the defendants own statements etc.
If the judge believes that there is literally no evidence at all that could create reasonable doubt then he won't give the instruction on criminal responsibility, so the jury will instead be told the defendant pleaded not guilty.
And then also the law states that the definition of a "mental disease or defect" does not include an abnormality manifested only by repeated criminal or otherwise antisocial conduct.
which would essentially rule out say a serial killer with a personality disorder, unless he had some other kind of mental disease or defect along with the personality disorder. He could still try the NGRI defence though but the prosecution will obviously keep emphasising how anti-social personality disorder does not satisfy the legal requirement according to the law and the judge will also be emphasizing the same in the jury instructions.
If not, then surely someone can claim something like intense anger or sadness or frustration meant they couldn’t stop themselves?
There's a provocation/heat of passion doctrine which is a mitigating circumstance that can reduce murder to voluntary manslaughter....but it must be the sort of thing that would cause a reasonable person to lose control, I would say the person may well have been temporarily insane at the time if something like that happens, but something like that doesn't count, it has to be the mental illness that caused the loss of control, not a circumstance.