To get into that discussion one would need to get into the differences between murderers, which you seem to be unwilling to get into, so I will just give an example, lets say that someone that has a kid and that kid was molested by a someone, due to corruption/lack of evidence that person is not condemned, and then the father goes and kills the child molester, in your view this person did this "Soon as you intentionally rob someone of their life you should forfeit yours.".
This person would undoubtedly be a murderer, But can you honestly not see how this person could be rehabilitated and become once again a productive member of society?
Not all murderers are Gilles de Rais.
Also really once again look at the murder rates per race, and you will see a vast difference between murders committed by minorities and murders committed by white people in the USA, one can either assume that this happened because white people are somehow genetically less predisposed to murder, or one can assume that there is a ton of social and cultural factors that lead to such a difference, which by extension does diminish (but not exclude) the individual role of an individual in the act of committing a murder, and if society can have such a tremendous impact on whether someone with a certain background ends more predisposed to killing or not killing another human being, one must also assume that said society can also impact those individuals in the other direction.
As for absolute guarantees they don't exist ever, much like you can't guarantee a drunk driver won't drive drunk again, or any number of other crimes, doesn't mean rehabilitation and reintegration should be excluded, you just simply take it on a case by case basis, and take the good with the bad, and the evidence we have of countries that have less primitive systems of incarceration point that the good far outweighs the bad.
