And it can be argued just the same that people predisposed to violence will be far more likely to do it again, "reformed" or not. What happens when they're in a situation when it's harder to control themselves (drunk, etc)? Just an example but it's appropriate here, and I know the obvious response is that they should know better, but really, if they couldn't control themselves once, would you honestly trust someone to compose themselves in the future, knowing that they're predisposed to doing it again?
What do you do about sociopaths? They're predisposed to crime and could similarly be found to be less liable for their actions too. What do you do about them? Do you give them a shorter sentence and release them because it wasn't their fault? On average, people using the insanity plea spend twice as long institutionalized. Pure sociopaths don't even get to use this plea, and the notable among them are sentenced to life or death.