Some critics of LotR have said that it is nothing more than pure good against pure evil, nothing but a comic book tale. However, I think that is disproved right away by Boromir and Denethor (and Gollum too); very complex characters who don't demonstrate either pure good or evil.

In the case of Boromir and Denethor they are powerful men who opposed Sauron. Boromir, for instance, was a fighter who led by example on the battlefield. For him to witness Sauron's continuing march to domination was particularly disheartening in the fact that he saw that his methods were no longer the ones that could win. That left him very susceptible to the pull of the ring. He welcomed this gift (which it seemed to him) could now make it possible for him to lead his people to victory through strength of arms. He would have the tools to fight against Sauron and win on "His terms." He was only redeemed when after the temptation was pulled away and his common sense came back to him. He realized that the ring wasn't the answer, and in his proud eyes he had failed the Fellowship and his people by trying to get the ring. However, he had in fact done the opposite by trying to save Merry and Pippin; and had gained redemption from the pull of the ring. Boromir died truly "good" and free from the rings peril.

In the case of Denethor, he had a much stronger will that had begun to peel away slowly after the death of his much beloved wife Finduilas. It was his strong-willed leadership that led him to believe he could control the palantir and use it for the good of Gondor. However the palantir led him to see too much for his own good, because Sauron was in possession of the Minas Ithil stone. Since Denethor was of Numenorean descent with a strong mind, Sauron couldn't control him directly through the palantir, and he couldn't lie to him. But he DID control his fears and the deterioration of his will. By the time of the War of the Ring, Denethor was a shadow of the great man he had been. All his hopes had been given to Boromir (Faramir may have reminded Denethor in mood of his lost wife) and when those hopes were lost after Boromir's death, Denethor's will was sapped completely.

He was a proud man who valued Gondor's rule and its history. He greatly desired to hand the rule of Gondor to Boromir and thereby continue the line of the Stewards. With the growing power of Sauron, however, it looked more and more as if that would be impossible even before Boromir's death. Added to the weight of the grief over the loss of Boromir was the fact that Denethor felt responsible because he sent Boromir over Faramir, feeling that Boromir better represented will, and the need of Gondor. As his will was sapped by the palantir and all Sauron would show him, he fell beyond redemption. When Faramir came back dying from Osgiliath Denethor felt that he had killed both his sons, and that his line (The line of the Stewards, which had lasted more than 1,000 years) was over. He felt that evil men would in the end rule Gondor and it drove him mad.

Tolkien was beyond writing characters that had no internal conflict. Arguably only Sam could be said to not have an internal struggle fueling their actions. All of the main characters in The Lord of the Rings are fully fleshed out, multi-dimensional characters. Boromir and Denethor are two characters who could never be called one-dimensional heroes. Their internal conflicts made them believably human and help the reader to realize Tolkien's world. All of Tolkien's conflicted and beautifully flawed HUMAN characters are just one of the aspects that puts Tolkien's incredible works above all the rest.