I could be wrong of course but I will share my take on not just this quote, but the DT and most of his writing. SK connects many of his stories to his others and many of them take place in Maine where he lives. I believe that much of this puts us on a much more personal plain with Mr. King than we realize. I think many of his characters are incantations of himself in different frames of mind, this may be why he decided to include himself in the story as well.

Now since we all have inner conflicts with ourselves I believe that what the man in black is saying is that he fears Roland succeeding because if he does than that one train of thought may rule with out prejudice and that is what he fears. He knows Roland is going to catch up with him and kill this form of he because this is ka but there are others like him or other forms of him that he wants to see continue after he is gone. So he does not fear death or bullets because he already knows he will die but he fears Rolands success.

Even in the ending King can't bring himself to let Roland win because he knows that to some extent this conflict is necessary to provide purpose in life. I may be streatching it now but I don't think the DT is done with Stephen and there will be more even if we don't get to see it.

That was a great question, hope I made some sense. Who's the HIPPIE now Brice? What?