1. Does the necessity of enforcing the law and ensuring compliance with the law justify extreme forms of punishment?
No. It’s not because you have committed a crime that you lose all your rights. The severity of criminal punishment should remain within certain bounds, and the need to be tough on crime doesn’t give you permission to do whatever it takes to be tough on crime. Most laws will never be respected in all cases anyway. A fetishistic attitude towards law enforcement isn’t helpful or necessary. Reasonably good enforcement is good enough. Convicting or deterring the marginal criminal is not a benefit that outweighs the harm done to the rights of criminals by the systematic imposition of extreme punishment (and extreme punishment has to be systematic if it is to have the required deterrent effect; punishing only one criminal in an extreme way won’t do any good, and some say that even systematic punishment has no deterrent effect).
2. If extreme punishment is not allowed, is it allowed to punish like with like?
Again, no, and for the same reasons as those given above. Lex talionis is unacceptable. Human rights are not conditional upon respect for the law, and the fact that punishment inevitably leads to some rights restrictions doesn’t imply that criminals lose all their rights.
3. But if criminals, by being criminals, don’t forfeit their human rights, how can one justify punishments such a incarceration or monetary fines which incontestably violate criminals’ human rights?
Those punishments can be justified, not as violations of rights but as limitations of rights. We need to limit the rights of criminals in order to stop them or deter them from violating the rights of others. In this respect, criminals are not treated differently from someone who yells “FIRE” in a crowd.
4. Is it justified to impose more severe punishments for the same type of crime on people who are more difficult to deter?
No again. Like the need to deter or stop crime doesn’t trump the human rights of criminals, it also doesn’t trump the rule regarding equality before the law.