A ruined life isn't much better than death, and sometimes worse IMHO. Without wanting to throw around accusations (truly! I don't mean to presume anything about you, specifically!), I find that people who make your argument often have no appreciation of just how terrible a ruined life can be. I think it's critical to remember that there can be no suffering after death, and while I'd still rather rot in prison than hang for a crime I didn't commit, there's no question that imprisoning an innocent person will cause tremendous suffering and debilitating handicap.
To be clear: there are other, stronger arguments against capital punishment. My point is rather that this one isn't the strong argument it's often made out to be.