Well i sugqest you shy away from a split personality, because if you are planing to be the leader you need to be sure in your cause. Also i think the origin of the taint should be from the drinking of blood. Your hate is for the elves and the undead; the elves because you hate something about them ( i will leave this up to you, but i suggest there efinity with nature ). I suggest you hate the undead, not because they're the source of your taint, but simply that you hate that people asosiate you with them when you are not. As for your character he will have to be a little crazy, coniving, and brutal. Not strait up evil, but instead willing to do what ever in the name of personal gain. So in some cases you will side with good, just for your own reasons.