[Belegorn]

One thing that may have contributed to Fëanor's actions was the 2nd marriage of his father to Indis. Elves marry for life and taking a 2nd spouse was sort of unheard of. This marriage in the end condemned his mother to being discarnate forever. To Indis and her children he held a grudge therefore. There is also this thing with the language of the Noldor. He was a linguist and he believed that þ should be used rather than s and when Indis married his father and accepted the Noldor's use of s rather than þ he felt that was a rejection of Miriel, his mother, because she used þ and so did Finwe until Miriel died. He had a lot going on. He thought his position as heir was going to be usurped, he thought the Valar were plotting against him, he hated Morgoth, and his father was killed by Morgoth.

Fëanor means "spirit of fire". This was his nature. He was in the end very much deceived by the designs of Melkor so I cannot put all the blame on him. The Silmarils were his greatest creation. Even the Valar knew not how he came to create such a thing. These were his darlings, his babies, his crowning achievement.

"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." [sil, p. 87]

In his speech to the Noldor he basically rehashed Melkor all over. He wanted to go to Middle-earth and rule there because he believed that the Valar were holding them captive in Aman so that a lesser people, Men, could rule the wide world. He wanted revenge on Melkor for stealing his Silmarils. He was not the only one deceived, but in the end his folly ended up making the Noldor a legendary people in their deeds on Middle-earth and in Beleriand.