Between close relatives there are always present all types of positive and negative emotions. The difficulty is that in insane person the slight marginal negative (or positive) emotion can get magnified and distort the perception and behaviour.
That way you can have the real emotions, based to some extent on reality, but completely out of whack. Add to it the memory recording the delusion or distortions as facts and you have a mess. Basing the prosecution on the statements of the psychotic person even if at the moment of the testifying the person is "sane" is very treacherous. You need a GOOD and VERY SMART psychiatrist to get the decent feedback but for the purpose of the law this information is USELESS.
Also since the onset of the psychotic episode can be gradual and close relatives lack the distance and objective criticism even the whole family can get confused and drawn into a pathological mindset for a while. Before they realise what is going on, some tragedy can happen. Even experts can make mistakes in such matters.
As well you can get the medieval trials through ordeals and be better off. The problem with the democracy that it lowers the complex problems to the level of the average intelligence which is not sufficient.
But, Andrea albeit a 'victim' was nonetheless the perpetrator of a heinous crime; and believe had she wanted to stop that morning and call for help; she would have, could have; she in fact did just that, after all five were drowned.
As to your conclusion:
["The problem with democracy (is)that it lowers the complex problems to the level of the average intelligence which is not sufficient."]
. . .do believe too much intellecutalizing can get one as far from 'Justice' as too little. . .think 'common sense' is sufficient; if not superior in most jury cases;
. . . problem is, it is not as common as it used to be. . .