I think it's reasonable to want truly insane people to be punished differently than sane people. Because law is not just punishment. It' s punishment aimed to both deter others from crime and rehabilitate the criminal. Insane people have different rehabilitation needs and their harsh punishment would not deter the insane from doing criminal acts, because they are, well, insane. A demented grandpa pulling a rifle shot into his wife because he thought he was back in the wars, and a husband murdering his wife to get revenge, for example, are good cases of the insanity defense being used as they should.

There are people who try to use that concept to get out of punishment. That has been true from the beginning of its use in modern law. Then again, it's the application of the law not the law itself that is evil. Corruption is already outlawed, I do not think outlawing the insanity defense is a good idea.