That's a critically important reason to oppose it IMO.

We've known for years that carrying out the death penalty is not even remotely cost-effective either.

And of course there have always been the moral arguments against it as well. 

We need to abolish it and be done with it as an issue.

It is extremely difficult to obtained a death sentence . A couple of mine were commuted a dozen years later on questionable bases to me. One died of natural causes. Another comitted suicide on Death Row. Only one was executed. I did not attend and was a bit squeamish. See a need for capital punishment in rare cases, but the process precludes any deterrent purpose. Many DNA reversals are based reasonable doubt not innocence and too much time has passed to retry them. The system is flawed, so doing away with it would not upset me that much.