It's perfectly fine the way I see it, with 2 minor things to worry about.
First, in order to have the BBEG be much more powerful then the PCs, but have them right on his tail, he'll be fighting about what they fight, but will barely level from it. So, you basically must know what the PCs levels will be when the final confrontation comes without making it seem forced or contrived when that time does come.
Second, he's a serial murderer, right? Why, if he's so much more powerful then the PCs, would he not just kill them right then and there? It's absolutely vital that you have a very solid reason for why this guy won't kill the PCs at the first chance he gets, and bad circumstances that he happens upon doesn't work for more then a few encounters before it seems unrealistic, and the Suspension of Reality starts to fade.