I've got this feeling that in the USA, where the death penalty is still available in most states, the murder rate is significantly higher than it is here. Whilst anything can be proved by suitable quoting of statistics, I don't think there is much evidence around that the death penalty has much impact on the murder rate - any more than imprisonment has much effect on the re-offending rate. I'd even hazard a guess that those who carry guns are much more likely to die by being on the wrong end of someone else's gun than they are to be convicted of murder themselves.