I have always been 'against' capital punishment.
My main reasons had to do with the view that killing people against their express wishes is barbaric, as well as with the fact that once a wrongly sentenced person has been killed, no subsequently discovered mistakes can possibly be rectified as the innocently executed person cannot be brought back to life. I was also always concerned that in the USA at least the distribution of death penalty suggests a legal system that is fundamentally unjust. Those most likely to get handed down a capital punishment verdict happen to belong to ethnic minority groups, and they happen to be poor. Rich murderers such as OJ Simpson on the other hand are able to buy themselves out of such verdicts by means of deploying legal teams capable of getting them off the hook.
Last but by no means least I considered the argument from the deterrent effect of capital punishment unconvincing, mostly because there had been no empirical evidence actually supporting this argument. This last reason is a truly important matter. If it can be shown that the existence of capital punishment incontrovertibly reduces the number of murders of innocent people one would at least have one powerful reason to reconsider this kind of punishment. It would then not seem any longer to be the case that a slam-dunk type case against the death penalty exists. One could still consider it barbaric, of course, and one could (one should!) have serious concerns about erroneous judgments, but these sorts of costs could well be outweighed by lives otherwise preserved thanks to the deterrent effect.
Well, you might want to review this particular issue. The New York Times today analyses recent empirical evidence and concludes that there is reasonably strong evidence supporting the claim that there is a deterrent effect. Even if you still think that capital punishment is an unconditional 'no no', you might want to reconsider your reasons for holding this view in light of the research reviewed in the NYT.