What we have generally agreed upon for our definition of evil is something along the lines of “imposing one’s will”, be it over other people or over objects. What is necessary for this to be the case is of course free will. If one is not enabled to make choices, to exert one’s will over one’s life and the direction it takes, then it is hard to say one’s will could be imposed on by others. What this line of reasoning leads to is that choices, the manifestation of free will, can go down the path of good, choosing what is best for the whole, or the group, or the path of evil, choosing what is best for the self. A heightened manifestation of this in the books is the conflict upon Amon Hen, when Aragorn is left with an impossible choice. He must figure out what is the best path to take for the remainder of the Fellowship, whether to pursue and join again with Frodo on the quest to Mordor or to pursue the Orcs and save Merry and Pippin from otherwise certain death. Aragorn spends much of the chapter “The Departure of Boromir” lamenting his inability to choose rightly, how the failing of the Fellowship comes down to his poor leadership, and generally going back and forth as to what the right decision would be depending on the circumstances. The careful consideration of this choice is seemingly meant to be important to Aragorn’s good qualities, his worrying over exerting his will unnecessarily and realizing the true weight of the decision that he has to make, and making it only as best he can.
Not only is this waffling uninteresting and even more aggravating to read than other repetitive parts of the Lord of the Rings, I would argue that the Battle of Amon Hen as depicted in the film version of the Fellowship of the Ring is far more thematically resonant while having not just Aragorn’s character be far more likeable and strong, but resonating with other members of the Fellowship. It was said that the book version was better because in the movie Aragorn “simply says that Frodo is no longer in their hands”, thereby reducing the power of his decision and making it have less weight. What this fails to take into account is the fact that Aragorn has already made the decision before this point. His decision was made at the top of Amon Hen, when he confronts Frodo after Frodo has fled from a warped Boromir and has seen Barad-dûr in a vision. Frodo is terrified of Aragorn, knowing that the Ring may corrupt him just as it corrupted Boromir. Frodo has decided that he must leave, take the Ring away from his companions so that they will not be dominated by the Ring. Frodo asks Aragorn if he can protect Frodo from himself, and if he would destroy the Ring. Frodo gives Aragorn the choice, either to take the Ring and use it or take the Ring and destroy it. But Aragorn chooses option three: he lets Frodo have the choice. He closes Frodo’s hand on the Ring and tells him that he would have followed him all the way to Mordor, implicitly telling him that Aragorn will honor Frodo’s decision to leave the Fellowship. Aragorn allows Frodo free will, and that is why this scene is far more powerful in the movie than the books.
But it doesn’t actually stop there. The entire sequence is made out of choices like these. Merry and Pippin have to make a similar decision, whether to stop Frodo or to let him go. Pippin is at first unwilling to let their Fellowship break, but he and Merry together make the decision to sacrifice themselves, distracting the Orcs so that Frodo may make it back to the boats undetected. Sam even has a way of implicitly allowing Frodo his decision. When Frodo says that he must go on alone, Sam says “Of course you are Mister Frodo, and I’m going with you!” This isn’t Sam imposing his will on Frodo, it’s an admission that both Frodo and Sam have made decisions and they cannot stop each other.
Even Boromir, poor Boromir, is allowed to show his true decision and redemption, an opportunity lost to the books. Boromir is not told by anyone to go and save the hobbits, but chooses himself to not go again after Frodo, either to apologize or to again impose his will and his love for Gondor on Frodo, but to save Merry and Pippin in their direst peril. He fights until his last breath, making his admission to Aragorn in the end of his mistake all the more meaningful. Boromir says he didn't have the strength of will to resist dominating Frodo, the strength that Aragorn did have. Boromir says that the race of Men is doomed if they don’t have that kind of strength, the strength to resist temptation and power. So when Aragorn tells Legolas and Gimli that Frodo is out of their hands, it is because he has already made the choice, tempting though it is to follow across the water, where a careful shot shows us Sam and Frodo just entering the forest on the other side. Aragorn had the strength to let Frodo leave, and that is at the heart of the entire sequence.
In the end, if what is evil is imposing one’s will on others, the very thing that the One Ring embodies, then it is the choices of the Fellowship upon Amon Hen in the movies that is the embodiment of good. The Fellowship breaks, yes. But it does not fail, as Aragorn rightly says, as long as they stay true to each other. Each of them have different decisions to make, and it is their goodness that lets them let each other make them, going on different paths towards the same goal of preserving the rights of the races of Middle-Earth to choose for themselves, and not be dominated and ruled by Sauron, or the Ring, or any other Dark Lord.
P.S. Also the sequence in the movie is just better written from an action, dialogue, and enjoy-ability stand point, but that's somewhat beside the thematic points.