It was Melkor who began this spread of lies among the Noldor. He told lies to Feanor concerning Fingolfin, saying thatFor Feanor began to love the Silmarils with a greedy love, and grudged the sight of them to all save his father and his seven sons; he seldom remembered now that the light within them was not his own.
And Melkor said also to Fingolfin and Finarfin that...Fingolfin and his sons were plotting to usurp the leadership of Finwe and of the elder line of Feanor.
This was how the pride of Feanor began, with lies from Melkor. However, he is only responsible for this first action, Feanor does all the rest on his own.Beware! Small love has the proud son of Miriel ever had for the children of Indis. Now he has become great, and he has his father in his hand. It will not be long before he drives you from Tuna!
Not only does he insult the Valar, he denies their right to the Silmarils, the light of which was created by Yavanna in the first place. Morgoth plunders the Silmarils, and returns to Angband with them.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...This thing I will not do of free will. But if the Valar will constrain me, then I shall know indeed that Melkor is of their kindred.
By this time Feanor has dug himself in too deep to get out. He and his sons leave Valinor forever, and commit the first Kinslaying as they leave.They swore an oath which none shall break, and none should take, by the name even of Iluvatar, calling the Everlasting Dark upon them if they kept it not; and Manwe they named in witness, and Varda, and the hallowed mountain of Taniquetil, vowing to pursue with vengeance and hatred to the ends of the world Vala, Demon, Elf or Man as yet unborn,or any creature ,great or small, good or evil, that time should bring forth unto the end of days, whoso should hold or take or keep a Silmaril from their possesion.