Feanor Self-Important?

[Lord of Angmar]

especially when there's the possibility that this person was attempting to accomplish the deed that Eru had purposed for him and put into his heart

Feanor should have known quite well that Eru would not put a purpose into his heart that conflicted so greatly with Manwe, who "in all things [has] served most faithfully the purpose of Illuvatar" (Ainulindale, 2nd Ed., 9).