Sorry mate, but operating a penal system on the basis that criminals will find Jesus and repent their sins is impractical and open to abuse. Some people need to be able to appreciate the consequence of their actions in the here and now, not just the hereafter. And whilst forgiveness by God is one thing, it doesn’t negate the need for people to be punished for their crimes in this world. Anything less is a recipe for anarchy if nothing else, and anyone who is truly repentant would accept the earthly consequences of their actions with good grace, rather than simply arrogantly assume that everyone else should just let it go because they claim to have found religion.
You are confusing what God has done and what man does. God, the definer of righteousness and justice, gave the world a blank check when His Son died for the sins of the whole world (1 John 2:2). Killing someone for an act that has already been judged and punished in the death of Jesus Christ is double jeopardy and, thus, unjust and unrighteous. The death of Christ 2000 years ago is a fact. Believing in Him or rejecting Him does not change that fact. So whether or not someone receives eternal life through Christ doesn't change the fact that his sins along with those of the whole world were already judged 2000 years ago.
So as far as any third party including the state is concerned, that man's sin's have already been judged and punished. Therefore, punishment is an invalid and unjust activity of the penal system because it is double jeopardy. That doesn't mean there aren't valid consequences. Valid consequences for an act and punishment are two different things even though the death penalty merges the two. Valid consequences enacted by the state and the penal system are incarceration of the criminal to protect citizens from the danger he represents to others. There are also other valid activities the penal system should engage in like attempting to rehabilitate who they can and making the incarcerated responsible and productive as much as possible to at least pay for their room and board.
Personal salvation and eternal life, however, is another issue and is STRICTLY between a man and God. No third party is involved. As far as personal salvation goes, "God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them" (2 Cor 5:19). God has done all the heavy lifting. God himself took care of the sin issue because it was too big for man to deal with. Sin is now no longer the issue. The issue is now Christ. What a man does with Christ determines that man's eternity. All it takes to be saved is cry out to God "that whosoever shall call on the name of the Lord shall be saved" (Acts 2:21). But even though it is easy to be saved, many refuse and don't receive Him. So even though Jesus died for my sins, I still must accept Christ as my savior to have eternal life.
So the state must deal with a man's acts but never with a man's beliefs. But the people of that state must realize the legal and judicial fact that God has already punished that man's sins on the body of Jesus 2000 years ago regardless of what that man believes (1 John 2:2). This proscribes punishment but not the duty to attach valid consequences to his actions.
Likewise, only God, not the state, can deal with a man's beliefs toward Christ. The state has nothing to do with eternal life of an individual. That is strictly between a man and God.