For the cheery world Gensokyo seems to be, there certainly are a lot of people who do horrible things. This thread is for listing them.
(Note that much of this requires funny interpretations of things. That's alright, this is a jokey thread anyway.)
Let's start this off with Yuyuko!
Yuyuko is a ghost. (Perfect Memento in Strict Sense)
Humans that spend time around ghosts quickly die. (Perfect Memento in Strict Sense)
Youmu, as a half-human, is affected by many of the things regular humans are affected by. (I haven't actually played IN Extra, but I hear something like this happens there for the Netherworld team.)
Thus, by keeping Youmu by her side, Yuyuko is slowly killing her gardener.
In PCB, she also mentions that "I've been inviting humans and youkai to their deaths just for fun. No problem." Were this actually the case, it would provide further evidence that Yuyuko is a monster. However, I suspect that's just a mistranslation/misinterpretation: given the flow of the conversation, it seems more logical that Yuyuko means that she can invite humans and youkai to their deaths just for fun, not that she actually does so.
(Also, she ate Mystia. Maybe.)