Good ol' ReiMari! I was confused as to what was going on at first but that's an interesting premise for a doujin, I hope there's more out there
The beginning reminded me of that one paradox in Forbidden Scrollery a lot of doujins focused on for a while where Reimu eradicates a human who has become a youkai, because it "defies order", which in turn means she would eventually have to confront Marisa when she becomes a witch
