Making the examples more extreme really doesn't change the argument, even though it probably does make it less comfortable, because of the emotions that such examples bring to the discussion. Obviously it's extremely difficult to forgive these people for what they've done, and to be honest if I were in the same position I'm not sure that I would be able to, but that doesn't mean that it's impossible or that doing so is unthinkable.
It's possible for someone to outgrow their murderous impulses, or get to the point where committing a crime is unthinkable-- child molesters can be castrated, murderers can be rehabilitated, addicts can get over their addictions. If someone changes so much that it's as likely that they'll commit a horrible crime as it is that someone else will commit a similar or worse crime, then punishing them is redundant... because again, the purpose of that punishment is supposed to be to prevent them from committing a similar crime, not to exact revenge. Of course, if the one who committed the crime refuses to or is too weak to change, then they shouldn't be released... but who are you to say that they don't just need more time?
One of the key steps to breaking one's psychological addiction to cigarettes is to identify yourself as a non-smoker as opposed to an ex-smoker... and even though you might not think there's a difference, it's actually a major one, and it can mean the difference between having frequent cravings for the rest of your life and having a complete non-reaction to cigarettes whenever you see them, or yes, even when you 'hit the wall' and feel like you need something to help you get through some horrible life event.
TLDR: People are neutral, identity is a construct that can, if necessary, be disassembled and reassembled completely, given time.