I don't know if someone has said this yet, but the punishment has to be removal from society, not death.
Justice has to be as fair as humanly possible, you cannot punish the person who is obviously guilty with NO shadow of a doubt at a disproportionate level to those that are found guilty beyond a reasonable shadow of a doubt.
First of all, it rewards the sneaky criminal, and makes it more of a crime to plead guilty... Secondly, with the simple fact that people do accuse and convict the wrong person all the time. when I moved to Washington state here in the USA, a man had just bean released from prison for a rape he didn't commit. The pictures of him and the actual rapist showed plainly why the woman who was raped made the mistake, at the very least they looked like brothers.
The only reason he was let free is because the rapist went in for a 30 year sentence on kidnapping, otherwise he would have stayed in.... If there had been a law allowing the state to castrate this man, his life would have been stolen from him in a bigger way than five years in prison did.
This happens to people, and personally, I see NO reason to allow the government to do more than sentence people to prison, remove them from society, of course! but if you take it to too far, then the state becomes the guilty party IMO. I would rather have a free society with a few problems than a society in which the government is privy to fascist ideas of law.
If you think prison is too easy or whatever, then spend a week in prison. It's not too long of a period of time. You'll be OK!
The law should never be an emotional issue, you have to make rational choices about punishment, or you become as bad as the criminals.
As a side note I was pro death penalty for the longest time after reading a few books on serial killers, but a few books on the nature of law made me rethink my views.