Watch out for the fake: if the perp is found guilty but suffers from diminished capacity AND life in prison, he goes to the loony bin until his shrinks proclaim him “sane”. Then he goes to prison for life. Problem, if found guilty of diminished capacity, but without a life sentence, he'll get released when his shrinks says he's OK to rejoin society.
If he goes to prison for life, he just may get the kind of justice he deserves at the hands of the inmates rather than at the hands of the Department of Corrections and justice system.