I'm not going to debate the theology of why God is not responsible for all the evil in the world in the Catholic understanding of the world (because hat is what this is ultimately given Tolkien's own background) since it would take too long and it is late for me
As for Sauron he was active and committed great acts of evil from the beginning of the World to the end of the Third Age. Feanor committed two evil acts and his sons a few more. It does not even remotely compare.
If you are not going to debate the point...
...don't debate the point.
Because there is TOTALLY a debate there we could be having, being on opposite sides of that issue.
Feanor committed more than simply two acts.
Blasphemy: Manwe told him not to go. Feanor told them to go hang themselves, and led his followers away.
Betrayal/Murder: Killing the elves to steal their boats when they refused to help him in his mad quest.
Theft: Stealing the boats for his mad quest.
Betrayal(again): Burning his boats once on middle earth so his followers now had no choice but to continue his mad quest.
Pride: Going off alone to follow Melkor because he couldn't be bothered waiting for the army his forced to follow him to catch up with him on his mad quest.
A Terrible Oath: When brought down by SUPRISEMURDERBALROGARMY, binding his sons to continue his mad quest and tolerate none would would not aid them...setting the stage for the ruin of the elves and middle earth through the next two ages.
TL;DR: Feanor's a dick. Totally as bad as Melkor in terms of how badly he screwed up Middle Earth.