It always seems like the demon reaper gets dumped into the story as a quick-and-easy antagonist, but the interesting ones are built around a core contradiction. They’re an agent of death, but they’re not necessarily evil, just profoundly indifferent to mortal concepts of good. That indifference is scarier than any axe-wielding maniac. I read a series once—can't recall the title—where a reaper kept being mistaken for a serial killer because the souls he collected were always in bizarre, violent circumstances. It wasn't his fault; he was just cleaning up the mess other supernatural forces left behind. The thriller came from this poor human detective trying to apply logic to an illogical world, and the reaper himself became this weird, unwilling ally.
Thinking about it, their best narrative role is that of a natural law or a force of entropy. They destabilize the human-centric view of the supernatural world. A vampire or a ghost still had a human life once. A reaper never did. That alien perspective creates a chilling distance, and a plot where a protagonist has to bargain with, outwit, or survive something that views them the way we view a leaf falling from a tree? That’s the good stuff right there.