Good and evil are pretty well-defined in The Fellowship of the Ring. Here's the gist: Ring = evil; no Ring = good. The crazy thing about evil in this novel is that is has the power to turn everything else evil, too. Even the most well-intentioned characters can be turned to the dark side by the power of the Ring. Something worth fighting against, we'd say. And except for Tom Bombadil, it seems like every character is in fact involved in the battle between good and evil: this conflict is the driving force in Middle-earth, and in Tolkien's story.
Boromir's fall to the Ring out of desperation to help Gondor suggests that any kind of strong personal attachment to a cause, even if that cause is a virtuous one like patriotism, may be twisted to evil ends. Ideology of any kind can become a root of evil behavior.
By conveying Frodo's unique suitability to carry the Ring because he does not want to and has little faith in his strength, Tolkien implies that the ultimate virtue in Middle-earth is not ability or loyalty alone; it is humility.