In Karthus' first lore, he was a lich, all alone in his tower in the Howling Marsh. But he didn't seem like the stereotypical lich that was just hellbound to destroy everything. His story seemed tragic. In one of the JoJ, he said something very interesting, lore-wise:
"In life, there was one worth singing for forever. I have kept my promise, though to some now the song is not as lovely as it was once meant to be."
I know the old lore is no longer valid, given the retcon. But the old lore is what I think fits him most, and this song is what I think can be descriptive of that: Dark Tranquillity - Her Silent Language (it's melodic death metal, so be warned~)
My interpretation of his old lore and the tidbits gained here and there, was that he was a mage who was extremely capable, a master of the arts. But his life was empty, till 'she' popped up. And since all good things end badly, she died, and he couldn't do anything about it. Be it disease, or murder, or anything else, he couldn't do anything, and no help was enough. So he started resenting life itself, and in undeath he found he could still hear 'her' voice, from the other side. He found a way to keep his promise, to always sing his Requiem. And blah blah blah .w.
Here's some referrence for a comparison between the lores: http://na.leagueoflegends.com/board/....php?t=2694261
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.