Now, as far as the Light part is concerned, there is nothing concrete. It is hinted that the Light is somehow conscious, for example it chooses who to instill itself into to create paladins in the Order of the Silver Hand. However blood elves managed to wrestle control of it from one of its physical vessels, M'uru. But then again, it turned out M'uru had allowed that to happen, to save the elves and re-ignite the Sunwell. Which may reinforce the theory rather than contradict it; and also explains why people such as the Scarlet Crusade can still wield it; perhaps it has a higher purpose, or just reckless writing. But it's all pretty vague. Which I like, since we don;t need explanations for everything in my opinion, and especially concepts that have something to do with deities. I like it when some mysteries remain secretive