I've seen before on Mumsnet that there's a tendency for people coping with MH issues (I fall into this trap myself) not to understand that there's a very wide range of MH problems.
I've never suffered from psychosis, but I have witnessed psychotic behaviour in the past. I witnessed my DB having such an episode, when we were on holiday in Africa. It was caused by the anti-malarial drug Lariam, which he should never have been prescribed as it was known to have a negative effect on people with MH issues.
He basically lost touch with reality and said the most bizarre things. Like telling my DM not to open a parcel when we got home, as it was dangerous. There was no telling him otherwise, as it was his reality.
So to this broken mum, the belief that she was the Messiah and she must sacrifice her son would have actually been her reality at that point. Hence the verdict of 'diminished responsibility'; she genuinely wasn't in her right mind. She was assessed a by psychiatrist and that was their assessment, which the court accepted.