I have a friend who has bipolar. They’ve been committed 3 times, twice voluntary, once by force. They’re diagnosed, medicated and 99.9% of the time function normally. They’re raising a family, very well educated, have a full time job etc. but they have flare ups triggered usually by stress which means that the meds become less effective. The 2 times they voluntarily committed was on the depression side of the curve, the time they were forcibly committed was on the manic side of their curve. They stopped taking their medication and things very quickly escalate. They went missing for a period of time, started posting stuff online that was so batshit crazy you just couldn’t believe the posts were from the person you knew. They’re lucky in many respects. They have a good support network around them in terms of family and friends and in the mental healthcare lottery they happen to have a postcode where mental healthcare is fairly decent by today’s standards, IIRC Nottingham ranks pretty low in that lottery.
All you've posted there is that you have a friend that has a mental health issue. One you've got a diagnosis you are a lot more privy to understanding your behaviours, what trigger them, and what you need to do to stay well. Of course flair ups can happen, of course things can go wrong. No one is also arguing that the cuts don't make things worse. The point is that there is line where responsibility goes to the person with the diagnosis. Whether that is therapy, cutting unhealthy people out of your life, keeping a stress-free job - once you are aware that you need to look after yourself, the person with the sole responsibility of that is you.
If someone is younger and has a manic or psychotic episode with no previous diagnosis or signs, you could argue that the defence might have half an argument. This isn't that though.
I really don't know how you're still not getting this. The man murdered three people violently, and tried to murder another three on top of that. He had a diagnosis, had help, was given medication, and on several turns refused to follow the necessary steps to stay well. As I said before, that isn't diminished responsibility, that's more or less the opposite. Your desperation to keep defending this man is disturbing.