Here is a good moral argument for why capital punishment should be banned:
If the convict in question harmed me personally by killing a member of my family, I would push that damn button in a heartbeat. And he should be thanking his lucky stars that I let the law take its course as opposed to taking it in my own hands.
We have passed the point where capital punishment is necessary to protect others from being harmed and is basically an instrument of revenge. I think Frodo captures much of what victims' families feel after such a tragedy and we then kill the murderer not for the public safety but our own thrist for blood. From a moral standpoint, it is not okay to kill the criminal using the law as your instrument just as it is not okay to kill the criminal via a hired hitman.