To be fair insanity is pretty much only a legal term these days. People with a psychotic, mood or personality disorder can be treated back to a normal or "pre-morbid" level in many cases.
As for someone like a serial killer whose been institutionalized rather than imprisoned they'd need a willing psychiatrist to say there's no chance of them re-offending, which is pretty unlikely as Doctors need to watch their arses in legal matters.
As for someone like a serial killer whose been institutionalized rather than imprisoned they'd need a willing psychiatrist to say there's no chance of them re-offending, which is pretty unlikely as Doctors need to watch their arses in legal matters.