LOTR: Why Is Gandalf Able To Help Theoden But Not Denethor?

gamerant.com:

One of the most successful techniques Tolkien uses in his writing is to use dichotomies within opposing characters, in order to demonstrate where their choices will lead. There are many examples throughout the Lord of the Rings trilogy, where two characters are presented with the same choice, and one makes the wrong decision, acting as a cautionary tale that shows the terrible devastation of this incorrect decision, while the other chooses the right path, and ends in triumph and celebration.

The best examples of this include Boromir and Faramir, both sons of Gondor, both human men who are presented with the opportunity to try to take the One Ring and use it to protect their people. On the one hand, Boromir succumbs to this terrible temptation, tries to take the ring from Frodo, and ultimately ends up losing his life because of it. On the other hand, Faramir has the same choice when Frodo ends up in his hands in Ithilien, but Faramir makes the right choice, lets Frodo go, and ultimately saves Middle Earth by letting the ring-bearer complete his quest to destroy it.

Why Didn't Gollum Become The New Dark Lord?

And it’s not just Men who are faced with these choices, these dichotomies go from the smallest beings (hobbits) right up to the most powerful beings (wizards). Another pair of opposites who show what giving in to desire and hubris means are Gandalf the Grey and Saruman the White. Saruman himself presents Gandalf with the choice, that they should join forces and serve the dark lord, and try to claim the ring of power for themselves.

At this point, Saruman has already fallen into darkness and his greed is beyond redemption, which results in him eventually losing Isengard, fleeing to concoct an evil scheme in The Shire, and