sunny dave - // To accept that they didn't understand the consequences of their actions is (essentially) saying that stupidity is a not just a mitigating factor, but a complete defence - which it shouldn't be. //
I think you have to be careful here not to confuse everyday speech expressions, with legally binding terminology.
I would suggest that the convicted couple did not avoid murder charges because they were 'stupid' - in my view, stupidity is the action of someone who is fully aware of what they are doing, but does not consider the consequences through simple lack of thought, which is not the same as being unable to consider the consequences.
I would also suggest that the couple have undergone appropriate psychiatric evaluations which have judged then as incapable of understanding the potential result of their behaviour - hence the charge or manslaughter, which involves an involuntary death, rather than murder, which involves an intended death.
// I'm unhappy with the circular argument that says "No sane person of normal intelligence could have done this - therefore they are automatically allowed the defence of insanity or limited mental capacity" - it's just not right. //
I would be too, if I believed it existed in law, which I do not believe it does.
The notion that 'no person of normal intelligence could have done this' is not one that is used to frame or enact the law.
Each case is assessed individually, and not on a blanket 'it can't be that, so it must be this' concept - that is not how the law works.