The way I interprete Tolkien, I think he meant Fëanor as an example how even the best among the best can be corrupted by their hatred. He does not mince words in regards to Fëanor’s positive attributes after all.

For Fëanor was made the mightiest in all parts of body and mind: in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike: of all the Children of Ilúvatar, and a bright flame was in him.