Instead of testing potentially harmful chemical products on animals, why not test them on people found guilty for rape, torture, and murder?

Criminals found guilty for rape, torture, and murder, are people who have violated other people's human rights in the worst way possible, and it is damage without repair. 

It is impossible for these criminals to mend what they have done, and nothing will ever bring the victims to justice. Yet, it is possible for the punishment of said acts to be carried out in the same manner as their crime: without mutual agreement. If it is known that this is the established punishment for said crime, it is possible for the rates of these crimes to decrease. If it is known that this is the punishment for said crime, the people who decide to commit the crime despite the punishment, are already agreeing to receiving this punishment, or at least willing to risk receiving it.