Two things.
I personally don’t like denying a person’s humanity as the basis for harsher criminal laws.
It may be negligible in this case, but is it possible to say it’s negligible in every case? If it’s negligible in this case, then he never gets out. But can we say that in every possible case, it will be negligible? Strikes me that the possibility of parole, even in extreme cases, is a continuation of the maxim for the presumption of innocence. “Better nine guilty go free than one innocent is convicted.” If there is a chance that even in some extreme cases a person may be able to rehabilitate, we need to keep that option open, while keeping those who can’t rehabilitate incarcerated.