I used to be a supporter of the death penalty. On an emotional level I often feel that some criminals should die....especially those who hurt children, or torture and kill mercilessly.
However, after taking a sociology class we learned that countries with a death penalty have higher murder rates than those without. It seems to be because the actions of the government are often internalized by the population as a normative ethical code. Therefore, if the government thinks killing people is a 'solution' then the general population will also think killing is a solution.
Once I thought about that, I realized that I actually do not support the death penalty.
Besides the circularity of the logic is well known: We kill people, who kill people, to prove that killing people is wrong.
Finally, there is a statistical probability that we accidentally kill innocent people, since our justice system isn't perfect. In my opinion, even killing one innocent person accidentally, is enough to make the whole practice immoral.
So, as much as I do think some people deserve to die, I don't think it is good to place the power of capital punishment in the hands of a government....particularly since no government is ever infallible.