Vin was the one who in fact killed the Lord Ruler. She discovered that he wasn't actually a god, or even immortal—he had simply found a way to extend his life and his power by making use of both Allomancy and Feruchemy at the same time. He wasn't the hero from the logbook—but, instead, was that man's servant, a Feruchemist of some great power. Still, he was much stronger in Allomancy than Vin. While she was fighting him, she drew upon the mists somehow, burning them in place of metals. She still doesn't know why or how this happened. With that power—and with the knowledge of his true nature—she was able to defeat and kill him.