Jesus stated in Nazareth, in one of His first recorded public addresses:
"He has sent me to proclaim freedom for the prisoners
and recovery of sight for the blind,
to set the oppressed free,
to proclaim the year of the Lord’s favor"
This is a little like a "Mission Statement", and, crucially, it states He has come "to proclaim freedom for the prisoners". It doesn't state, "freedom for those wrongly imprisoned", but freedom for prisoners, indiscriminately, rightly or wrongly imprisoned, no matter what they did, no matter what they deserve.
It doesn't mean freedom in the sense of just letting people out randomly - for what freedom is a life of crime and desperation? It means freedom in both a physical and an emotional sense, a freedom to be accepted, loved, and find new, transformational hope in a future of goodness God has for them. It means our primary aim in 'criminal justice' systems ought to be to rehabilitate and reform offenders, which can happen to anyone by God's grace, and thus proclaim true 'freedom' to them.
This is a primary reason why the idea of the death penalty is such an absolute travesty, flying in the face of the grace Jesus proclaims. It may seem obvious, but killing someone is NOT proclaiming freedom to them. Putting a prisoner to death does NOT fulfill Jesus' mission to "proclaim freedom for the prisoners". This is not a difficult question to answer; it is categorically true that killing someone is not the same as proclaiming freedom to them... Rather, it's utterly giving up on them in the worst way possible. Inhumane prison conditions, maltreatment, or torture, also do not proclaim this freedom. All these seem to be failures of love, rejections of grace, and, I believe, frankly, insults to the Gospel.
"Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners—of whom I am the worst. But for that very reason I was shown mercy so that in me, the worst of sinners, Christ Jesus might display his immense patience as an example for those who would believe in him and receive eternal life." (1 Timothy 1:15-16)
If Christ Jesus came into the world to "save sinners", even the "worst", what is our business ending their lives? What is our business cutting off their opportunity to receive Jesus' salvation? To use an analogy, since when did blowing up a lifeboat full of people count as a legitimate way of "saving" them?
No-one is too corrupt to change. The example of Saul shows this definitively - why else did God choose him? It was for the "very reason" that he was the "worst of sinners" that God chose him. As such, without a doubt, no-one is beyond the reach of God's love and potential for a new life, no matter how many mistakes they have made. We must put this into practice, and not insult God's transformational grace by killing people before they have a chance to reform.
In such a way (and for many other ethical reasons), I am utterly and wholeheartedly opposed to the Death Penalty.