[Gothmog]

How about Gwindor? He started off as a great elven-lord, but then his brother Gelmir was caught by Morgoth and killed in front of Gwindor's eyes (just before Nirnaeth Arnoediad). Then he charged forward, the troops followed him and that unprepared attack was one of the things that led to the disaster in the battle.

He was than imprisoned and turned into a slave in Angband. He managed to escape but was now broken and completely changed, people didn't recognize him anymore. He met Turin and they went back to Nargothrond, where Finduilas, Orodreth's daughter fell in love with Turin instead of Gwindor (Finduilas and Gwindor were in love before Nirn. Arn.). The elves of Narg. listen more and more to Turin and less and less to Gwindor, who finally died at the battle of Tumladen, a battle which Turin more or less was responsible for.

From a mighty warrior and lord to a broken ex-slave who lost his love and many friends, much because of Turin. (of course it wasn't Turins intention, but that curse of his...it was bad )