life prisoners and medical advances that prolong life

[Antonio the Sixth]

I don't think your distinction between "interactions between individuals" and social sanction makes much sense, honestly. Society is made up of individuals, and the interactions between these individuals is what creates social norms and customs. Criminal sanctions are only a subcategory of that. It really doesn't make sense to tell people to be merciful to each other in one-on-one interactions but at the same time draw up draconian laws that don't leave any room for forgiveness.

As for your second point, you began this discussion talking about people "with a pathological and nearly irresistible desire to inflict physical harm on others". It sounds like you're talking about something else now. Again, pathological sadism can and should be treated (even if of course that doesn't mean it can always be "cured"). If you're talking about the garden variety of rational, mentally sound evildoers, then yes, I do believe that at some point all of them deserve a second chance.

[RFayette]

I typed out a long response but I got logged out, but I think the word "incorrigible" is better than "irresistible."  The person would still have agency, but his desires have become so twisted that even though he fully could choose not to commit a crime, he has no interest in doing so. 

As far as forgiveness goes in a penal system, I think there has to be a balance, but the interests of those who obey the law have to take precedent over those who choose to break it, especially when it comes to those who are repeat offenders - it's not just the offender and the victim here, but also all of society who could be potential future victims that needs to be taken into account.  The deterrent effect of punishment in others is also an important factor at play, and I think these distinctions are necessary when we decide what punishments are appropriate.