Do not repay anyone evil for evil; be concerned for what is noble in the sight of all. If possible, on your part, live at peace with all. Beloved, do not look for revenge but leave room for the wrath; for it is written, “Vengeance is mine, I will repay, says the Lord.” Rather, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals upon his head.” Do not be conquered by evil but conquer evil with good.
-Romans 12: 17-21
Repaying evil for evil is essentially what vengeance is. To apply this to capital punishment you have to presume that the only reason why a state would execute someone is to wreck vengeance on them. Then, I suppose, it would follow from this presumption that the only reason a state would sentence individuals found guilty of any offense whatsoever to punishment is because the state only seeks vengeance and the repaying of evil with evil. That sounds to be a bit of an odd claim.
Or are you claiming that the only reason anyone would argue for capital punishment is from a desire to “repay evil for evil?” That would mean you have dismissed all other arguments for capital punishment as irrelevant or unconvincing and have found that this “vengeance” argument is the only one which you think to be at all persuasive and the only one necessary to refute.
Unfortunately for you there are far better arguments for capital punishment, and vengeance, as far as I know, has never been seriously offered by any serious debater.
Nice try, though.