I oppose it even for the correctly convicted because I value their lives.
For those who have different values, there are two other reasons to oppose it.
1. The 'an innocent might be executed' issue. In fact, it's pretty clean innocents have been executed. Many people feel 'if one innocent is executed, it's not worth it'.
Not that life in prison for an innocent person is not a horrible thing.
2. The cost. The figures I have are that it's about three times the cost on average to execute someone as to put them in prison for life.
This is basically because of legally required additional legal proceedings.
Now, some ignorant people like to respond to that with cowboy arguments - 'I'll do it for free' and 'we need to cut back on those riduclous protections for criminals'.
While I'm not going to get into the reasons those protections are needed here, the relevant issue to cost is that these are fundamental Supreme-Court mandated protections. They can't be 'just removed' because the tea party elects some nuts who want to take away rights from the criminals (read, you wrongfully convicted).
So, the people who would still support capital punishment are those who want to execute people who don't value human life as much, who value the revenge more than they are concerned about executig innocent people making themselves murderers in effect, and who are happy to spend large amounts of tax dollars for that revenge.
That would be if they understood the issue - I don't think many of them do, thinking execution is cheaper and just deciding on the emotion of hating the criminals.