If you’re in the UK at least, a mental health defense will not get you off with a lighter sentence.
You’ll probably end up with an indefinite sentence (ie you can only be released when it’s judged by a team of psychiatrists that whatever was wrong is sufficiently sorted that you are no longer a danger to the public).
You will also go to a secure mental health facility rather than a prison. The conditions might possibly be better (it depends which one you get sent to vs which prison you would’ve been sent to) but you’re going to be surrounded by other people who are at best challengingly to deal with above and beyond normal prisoners and will probably be taking a fairly strong regimen of medication that absolutely will have side effects.
I’ve been to an acute psych ward this year. They’re not places you want to spend time. I can only imagine the secure versions being significantly worse in every respect.
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