Just to add a new twist to the argument, during the London Riots a number of street gangs teamed up to commit acts of criminality. A lot of them were arrested and given long sentences - whilst they've been off the streets, crime in those areas has gone down.
So here is my pro-capital punishment stance - if the criminal shows no remorse for their violent behaviour, then put them down, the same way you would a wild animal. They cannot / will not be rehabilitated due to nature / nurture and their continued existence is a drain on society.
(Though I'd rather have them locked up for long periods of time and off the streets)