@swbarnes2 has the beginnings of a great answer, but it needs to be take much, much further.
In Tolkien's world -- as in orthodox Christianity -- one of the greatest evils that can happen is the corruption of good, and both Lord of the Rings and the Silmarillion are full of such falls. The fall of Melkor, was terrible and so were the lesser falls of Saruman and Denethor (and dozens of other characters). Gandalf and Galadriel both fear the Ring because it would cause themselves to distort their own greatests strengths and turn them to evil. (Quotations available upon request!)
Craftsmen are most likely to fall by becoming too involved with or possessive of their creations. Who were the craftsmen who fell? Sauron (originally a Maiar of Aule the Smith), Saruman (likewise), Feanor, and many, many Dwarves. First, look at Aule, a smith sho did not fall:
...but the delight and pride of Aule is in the deed of making, and in the thing made, and neither in possession nor in his own mastery; wherefore he gives and hoards not, and is free from care, passing ever on to some new work.
He was a creator who was not trapped by his creations.
Feanor on the other hand,
...grew swiftly, as if a secret fire were kindled within him... in the pursuit of all his purposes eager and steadfast. Few ever changed his courses by counsel, none by force. He became of all the Noldor, then or after, the most subtle in mind and the most skilled in hand. ...he it was who, first of the Noldor, discovered how gems greater and brighter than those of the earth might be made with skill. The first gems that Feanor made were white and colourless, but being set under starlight they would blaze with blue and silver fires brighter than Helluin; and other crystals he made also, wherein things far away could be seen small but clear, as with the eyes of the eagles of Manwe. Seldom were the hands and mind of Feanor at rest.
So far, so good. He was proud and headstrong, but still unfallen. But then he made the Silmarils:
Feanor, being come to his full might, was filled with a new thought, ...and he pondered how the light of the Trees, the glory of the Blessed Realm, might be preserved imperishable. Then he began a long and secret labour, and he summoned all his lore, and his power, and his subtle skill; and at the end of all he made the Silmarils.
As three great Jewels they were in form. But not until the End, when Feanor shall return who perished ere the Sun was made, and sits now in the Halls of Awaiting and comes no more among his kin; not until the Sun passes and the Moon falls, shall it be known of what substance they were made. Like the crystal of diamonds it appeared, and yet was more strong than adamant, so that no violence could mar it or break it within the Kingdom of Arda. Yet that crystal was to the Silmarils but as is the body to the Children of Iluvatar the house of its inner fire, that is within it and yet in all parts of it, and is its life. And the inner fire of the Silmarils Feanor made of the blended light of the Trees of Valinor, which lives in them yet, though the Trees have long withered and shine no more. Therefore even in the darkness of the deepest treasury the Silmarils of their own radiance shone like the stars of Varda; and yet, as were they indeed living things, they rejoiced in light and received it and gave it back in hues more marvellous than before.
All who dwelt in Aman were filled with wonder and delight at the work of Feanor. And Varda hallowed the Silmarils, so that thereafter no mortal flesh, nor hands unclean, nor anything of evil will might touch them, but it was scorched and withered; and Mandos foretold that the fates of Arda, earth, sea, and air, lay locked within them. The heart of Feanor was fast bound to these things that he himself had made.
..For Feanor began to love the Silmarils with a greedy love, and grudged the sight of them to all save to his father and his seven sons; he seldom remembered now that the light within them was not his own.
My emphasis: Feanor had fallen into a pit of his own devising and fallen in love with his own creations.
He has a chance to save himself after Morgoth and Ungoliant kill the Two Trees, and Yavanna says that with just a little of their light from a Silmaril, she could call them back to life:
Yavanna spoke before the Valar, saying "The Light of the Trees has passed away, and lives now only in the Silmarils of Feanor. Foresighted was he! Even for those who are mightiest under Iluvatar there is some work that they may accomplish once, and once only. The Light of the Trees I brought into being, and within Eä I can do so never again. Yet had I but a little of that light I could recall life to the Trees, ere their roots decay; and then our hurt should be healed, and the malice of Melkor be confounded.'
Note that Yavanna said without qualification that only a little of the light would be needed. She asked for a single Silmaril.
Then Manwe spoke and said 'Hearest thou, Feanor son of Finwe, the words of Yavanna? Wilt thou grant what she would ask?'
There was long silence, but Feanor answered no word. Then Tulkas cried 'Speak, O Noldo, yea or nay! But who shall deny Yavanna? And did not the light of the Silmarils come from her work in the beginning?'
But Aule the Maker said 'Be not hasty! We ask a greater thing than thou knowest. Let him have peace yet awhile.'
But Feanor spoke then, and cried bitterly 'For the less even as for the greater there is some deed that he may accomplish but once only; and in that deed his heart shall rest. It may be that I can unlock my jewels, but never again shall I make their like; and if I must break them, I shall break my heart, and I shall be slain; first of all the Eldar in Aman.'
Feanor still refused. At that point none of them knew that Morgoth had already stolen the Silmarils and they had been lost ot Feanor even before her refused to help. The narrator notes:
The Silmarils had passed away, and all one it may seem whether Feanor had said yea or nay to Yavanna; yet had he said yea at the first, before the tidings came from Formenos, it may be that his after deeds would have been other than they were.
Feanor, like many other characters in Tolkien's world, fell through his own strengths and by the particular failing of the craftsman of falling into a possessive love if his own creations. (Well, he's also a bit of a drama queen...)
