Boromir
An Example of Twisting of the Good
From C.S Lewis' The Screwtape Letters
And
J.R.R Tolkein's
The Lord Of The Rings

One Ring to rule them all
One Ring to find them
One Ring to bring them all
And in the Darkness bind them.

The Ring of Power was an evil, corrupting weapon. It twisted everything which was good in the beginning into something horrible to serve the Enemy. Boromir of Gondor, one of the nine companions that journeyed from Rivendell (the house of Elrond, the Elf lord) fell prey to the Ring's temptation and all of his good characteristics were used against him. This is exactly what Screwtape in C.S. Lewis' The Screwtape Letters describes as "Twisting of the Good." Screwtape knew that evil can be snuck into all places of a person's life, till it pollutes the virtue and makes evil part of the will.
Boromir was a man of courage and loyalty, and he loved his country and would do anything to save it and its people. When he was present at the Council of Elrond, the Ring of Power clearly affected him; he immediately desired it, but he was chastised by Elrond and was figuratively awakened from his temptation. However, the desire for the Ring was placed in his heart by the enemy, to corrupt his mortal mind unless he was strong enough to resist it. Boromir suggested using the Ring for good purposes, but he didn't realize that since the Ring was completely evil, it could only tempt and corrupt good things.

Boromir joined the Fellowship of the Ring, which was the group of beings given the task of taking the Ring to Mount Orodruin to be destroyed in the fire there. On the journey, Boromir slowly fell prey to the Ring's influence. His actions became questionable, and it was very easy to see that he no longer desired the Ring to save Middle Earth, but only to take it for himself and wield its power alone. He still believed he could defeat the Dark Lord by using the Dark Lord's own evil weapon. When the Fellowship crossed the pass of Caradhras, he grabbed the Ring from the ground where it had fallen and almost touched it, before he was startled by another member of the Fellowship. If Boromir had touched it, he might have been completely tempted by the Ring and taken it for himself. This type of action is noted by Screwtape when he describes sin as being harder to defeat once you have succumbed the evil. Boromir reluctantly handed it back to the Ringbearer, Frodo, and acted flustered but careless, as if he didn't want to admit he had been tempted again. When the Fellowship arrived in Lothlorien, he was terrified of Galadriel, the good White Lady of the Wood. She spoke to Boromir's mind, and he knew she could feel his desire for the Ring and his corrupted spirit. She warned him of the evil in his heart, and perhaps for a time, while the Fellowship remained in Lothlorien, he repented of his temptation and tried to fight against it. Through all of his trials, Boromir was still a good man and had a pure heart. He still fought to keep the evil away from himself. This is an example of  Screwtape's point about evil gradually moving from virtue to will. Boromir strove to keep the evil out of his will as much as possible.

The last part of Boromir's journey with the Fellowship put him to the ultimate test. Frodo wandered off alone to think, and Boromir secretly followed. He attempted to persuade Frodo to give him the Ring freely, but Frodo realized that it wasn't Boromir talking, instead it was a corrupt power that had taken over Boromir's character. Frodo refused to give up the Ring, and Boromir tried to take it by brute force. He struggled with Frodo and allowed the temptation to completely take over his mind. Frodo had to use the Ring's power to become invisible to escape, and Boromir realized what he had done and cried out to Frodo, sobbing his apologies. However, Frodo was gone, and Boromir returned to the other members broken and confused. He could not explain his deed, however, because the Fellowship was suddenly accosted by Uruk-Hai, a devilish breed of orcs. Boromir fought valiantly, protecting the two young members of the Fellowship until he was killed by the orc captain. Aragorn, the other Man on the Fellowship, came to Boromir's side right before he died, and Boromir revealed all of his sins against Frodo and the evil in his heart. Boromir makes Aragorn promise to do what Boromir could not, and that is make sure the Ring was never used again for evil. He died, and Aragorn never told anyone else about Boromir's confession.
J.R.R Tolkien used the concept of Twisting of the Good when he created the Ring of Power. All the Ring could do was take what was already good and twist it for its own designs. It could never create something only out of evil. This part of Evil is shown clearly in all of Tolkien's writings about Middle Earth. From the very beginning, the main demon Morgoth could only twist the good song that God (Or Eru Ilúvatar, The One Father of All) had created, he could not create his own song. Satan is the same way. He can never create his own evil. That means that all bad things in this world today are only corrupted forms of what was once good and perfect; gifts from God. There were other characters in Tolkien's books influenced by the Ring, but Boromir was unique because of his courage and will to stand against the temptation, even to death. Though the Ring did ultimately cause his downfall, he still let go of the Ring at last, doing what others could not.