I'm opposed to the death penalty not because I don't think there are some crimes so heinous that they merit death as a punishment. I'm opposed to it because I don't think the government is capable of administering it fairly, competently, and with adequate protections to prevent the execution of an innocent person.
This is an issue that I have moved pretty far on since my high school conservative days. I used to be a death penalty hawk -- I suppose this was in part due to the natural tendency to take the opposite side of folks making bad arguments. Death penalty opponents would argue that we just don't have the right to take away the life of that lady who drowned her three kids by sinking them in a car in a lake because she was tired of taking care of them. Well, I felt she had pretty much forfeited her ability to fall back on the sanctity of life defense.
But I am increasingly pessimistic of the justice system's ability to adequately separate guilt from innocence (it is run by the government, after all). We have far too many examples of people who have exhausted their normal appeals and have sat in jail, and even on death row, for years or decades before exculpatory evidence came to light (or, in situations of bias like in the deep south, where courts were finally willing to consider exculpatory evidence). We can only tremble to think of how many innocent men were never cleared before the day of the fatal injection came. Prosecutors, who often are using the position as a springboard for higher office, generally have the incentive never to back down from a case and to defend every conviction, no matter how clear the evidence becomes that an innocent person is in jail, to the very end (see Janet Reno, for example, who in a twist of terrible irony now sits on the board of the Innocence project, while men falsely convicted in her day care pogrom still sit in jail).