I got a bit confused at the end. Was the implication that Reimu is secretly eating humans (using the excuse that those humans tried to become youkai) to eventually turn herself into a youkai too? And Marisa is in on it? Or is it something else?
I think Marisa is helping turn humans into youkai to make sure Reimu stays in her Hakurei role by exterminating them. Similar to how Yukari is also making sure Reimu stays in her role albeit in a different manner.