
I think there's something you forgot. Morgoth never leaves his castle angband in all wars. So he doesn't single kill elves. He only comes out to duel with fingolfin. If your answer to the question is glaurung, please take a poll by voting. Thank you.
I also feel like Sauron was involved very much in that. Being his greatest lieutenant, it would not surprise me. That's why I picked Sauron.