I've got a gut feeling that this is somehow isn't really as dark or malicious as it looks. Like, maybe these dreams are a manifestation of the suffering that people are bottling up as they sacrifice their own happiness for the sake of others. After all, "Yamada" is used as a sort of everyman name in Japan, as far as I'm aware.
Also, MC has the actual most common surname in Japan, which might imply a deeper connection between him and "Yamada."