You don't take away what the first man deserves, withholding his pay because the second man didn't get paid. That would double the injustice. Innocent people get condemned. This is a criticism of any system of justice, not a particular type of punishment. Life is flawed, not capital punishment.
Guilty people repeat crimes that injure and even kill other innocent people.
But death can't be undone. No punishment can be undone. Our attempts at improving justice here must be at the level of the process of adjudication making any determination of guilt more trustworthy. Our case is not for every one's life or every form of life. Pro-lifer's are against the unjust taking of innocent human life, particularly the life of the unborn child.