There isn't a set one. It depends on the type of ill-treatment. It will be similar to the punishment for the same ill-treatment of a non-mentally ill person but possibly more severe.
Well according to a psychiatrist I spoke with personally a while back it is possible to have a 'mass psychosis' where two or more people experience the same symptoms as each other. I personally think...
It depends because murderes might of had a rough past with there family and they just wanted to let all of their hatred out, or they just do it because they ar ill.