No one has proved that killing murderers/rapists etc stops others from going ahead and making these crimes. When countries (such as Canada) got rid of the death penalty, there was no increase in crime. Resent studies in California failed to find that death penalties have a deterrent effect on criminals.
I believe that the death penalty is an escape route for criminals anyways. With being put in a cell for life imprisonment I believe they suffer more. Guilt can overcome them.
It is not right that different countries have different rules. If there is going to be a rule for capital punishment, it should be the same the whole way round the world. It is not fair that someone in England can kill someone and not die themselves, whereas in Singapore that's a different matter, and the murderer is given a death penalty.
But it is not really the criminals who suffer the death penalty. What about their families and friends who have to go on living their lives.
Although these people have committed a serious crime, in reality there is no other explanation for taking their lives. Yes they have done bad, but after all, all killing is murder. And on these grounds I believe capital punishment of any kind should be abolished.