Being a Hakurei shrine maiden is stressful. You don't know who you might end up having to kill. It might be a total stranger, or it might be...
It just occurred to me that if Marisa were to get the Jinyou treatment, she might just be another best friend Reimu had to put down – and not the first one. Expanding on that, I think it's likely that quite a few in Reimu's body count were her close friends. Maybe the most recent before eventually Marisa was Rin.
