So far the deterrent effect of capital punishment seems to be entirely theoretical. I see ethical problems with imposing the ultimate penalty based on what someone thinks the result will be.
Secondly, there is a serious moral issue with imposing an irreversible penalty given the existence of people who are wrongly convicted. My uncle, for example, was convicted of a murder that someone else later confessed to. If he had been executed, there wouldn't have been a thing anyone could have done about it.