we've all likely heard of cases where someone on death row for years was proved innocent by the relatively recent ability to test for DNA. i know of a man in my area that was convicted by testimony of several witnesses and sentenced to be executed, only to be proved innocent years later by DNA testing - and the actual murderer confessed to the crime once the DNA evidence pointed to him instead of the man wrongly convicted.
it is clear to me from the New Testament - the scriptures i've offered as well as others - that the death penalty is not for today from the perspective of Christianity. Even if you have a different understanding of New Testament scripture on this subject, there can be no denying that innocent people have been executed by mistake while murderers went free due to the mistaken conviction of the innocent.
since we can't agree on what the New Testament teaches about the death penalty, the core of this subject lies in who is qualified to judge and condemn - and i don't see how any Christian could claim absolute correct judgment (much less the unsaved judges in our court system) - if no one is qualified to judge absolutely accurately without error, then no one should be executed. someone sent to prison by mistake can have a life once the error is corrected by the justice system; but there is no correcting a mistaken judgment that results in execution. 'when you steal from a man, you take away what he now has in this life, this can be returned to him; but when you execute a man, you take away all he will ever have in this life' (rough quote from a movie)
as far as taking someone's opportunity to be saved by executing them, I find it impossible to believe that the God who is Love would allow anyone to go to hell without more than enough opportunity to decide for themselves. those who go to hell will do so only after struggling over countless obstacles of Love that God has placed in their path; i don't see that executing someone condemns the judge - this was also paid for by Jesus - He paid the price of sin for the WHOLE world. what sends people to hell is rejecting the God who has already paid the price for sin; not the 'sins' people commit here in this life - they have already been paid for.
Romans 2:1 KJVR
(1) Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.