every being on this planet, perhaps in this universe, has the PotentiaL, to make a positive contribution. [yes evil negative contribution as well]
some willingly and ready, some after tempering, some unwilling but subject to management/redirection.
when you end a life, you end its active potential, and relegate the contribution to those affected by the potential.
melkor sought to destroy the machinations of illuvatar,but only made something of greater beauty as a result of attempted destruction.
frodo, and sam having a sense of unseen potential could not deliberately destroy gollum, for his evil.
this is why destruction rather than mitigation, is a harsh decision of last resort.