Can't blame the marisa, just look at that tongue, it isn't only a retard but has some probability of deibuismn, and we know how serious that "illness" is!
It does seem a little harsh to outright squash her, but if they were to let her live on, while I don't know how severe her impairment is, I think she would have to heavily depend on others to get even the simplest of tasks done. And living in the yukkuri world, being a retarded yukkuri could really only lead to one thing.
chomp
scarfbwuuurp...chompscarfI've wanted to talk to you for some timeIt's for her own goodWe should squash her immediatelyWhat are you talking about? MarisaThis little one can't take it easyYu? What is it, MarisaYuja! Yujii!Yuyuji!Yucchichi!...Yufufu! Eat a lot, little one!YuYucchi!...YuMarisa too! Let's munch munch easy!...I'm going to squash her...ReimuLittle one! Mister food!Yu!