Where is it clear that we execute innocent people? To my knowledge not a single executed prison has been posthumously cleared since the death penalty was brought back in the 70's.
Give that 129 people have been released from death row due to having been found innocent in the same time, why do you suppose this is?
(1) The system really is perfect and no, or a negligible number of, innocents have been executed?
(2) There's very little interest, resources, and money being devoted to investigating crimes for which the culprit has already been put to death?
For the anti-DP crowd, this is the Holy Grail against the death penalty. If an innocent person had been executed, they surely would have found it by now.
How, exactly, would the go about doing that? They'd have to find the evidence, find an attorney willing to take on the case (or pay one to do it), and run through an extremely expensive trial -- on behalf of somebody who's already dead, and whose family is very likely indigent or at least unwilling to dredge up an extremely painful experience to start with. They'd also be facing a prosecutor's office who would fight tooth and nail for their reputation -- being found to have executed an innocent is very bad press, even among the pro-DP crowd.
The "anti-DP crowd," as you call it, has certainly found plenty of evidence. There just haven't been any trials.
Links about these cases, and about the difficulties involved in reopening cases where there has already been an execution:
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http://www.democracyinaction.org/di..._KEY=2489&t=Innocent And Executed Section.dwt ]
[
http://www.commondreams.org/headlines05/1121-05.htm ]
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http://www.deathpenaltyinfo.org/article.php?&did=2238 ]
...and, yet again, this is one discussion where I'm bowing out at this point. If America chooses to kill its own citizens, with or without due process, it's really none of my business to try to argue the case, one way or the other. I've stated that I deplore the practice, and should really have left it at that.