First, yes, PG is right that we need to do something about wrongful convictions. If we do away with the death penalty and these people live the rest of their life in prison, but we don't kill them, that's... better, I guess? But it's still horrible, and it's still a problem that we need to fix. Just eliminating the death penalty doesn't fix the problem. If you make eliminating the death penalty a side effect of the campaign against wrongful convictions, that's fine. But if you make eliminating the death penalty the fix, that's hopelessly inadequate.

Second, the moral calculus gets awful. The usual question is, how many guilty people would you let go free to avoid putting one innocent person in prison? That shows the problem - false positives and false negatives are inversely related. But there's another issue, which is that some people who murder do so more than once. So, to avoid putting one innocent person in prison, how many innocent people are you willing to see die because of not-convicted murderers who repeat the crime? (If anyone has recidivism statistics on released murderers, I would welcome them.)