Why is killing by execution legal and not a considered violation of human rights while beating and whipping is?

I think killing is worse than beating or lashing.Saddam Husein's Defense Lawyers claimed he was physically abused and beat ed while is custody and they say he had marks on his skin but hanging him and suffocating him is ok why? Why do criminals have rights not to be beaten but can be killed?I think corporal punishment should be legal. In this country some rapist get sent to prison for 25,30,40 or 50 years but I think lashes would be better so they learn not to do it again not stay in prison until they are in their 70s or 80s.