Doctor thinks Harold's gonna die soon due to the cursed sword, because he fully believes Harold has been using it more than in actuality. Reincarnator inside Harold knows about the sword's properties, though, and has been deliberately using it only as absolutely necessary, so he's got a lot more life in him than he's letting the doctor believe, but less than he would if he never got the sword at all. Beyond that, the doctor is well aware that Harold hates his guts and knows more than is letting on, and presumably has other contingencies to ensure Harold can't just off him -it's one part of why he keeps Harold running around across the nation rather than at his side. So Harold's trying to push the story along with just enough differences that he lives at the end, but the hero's party still takes out the doctor like they're originally supposed to.