Why does tanako kill everyone in higurashi no nako koro ni?

Answer 1

Because she has a messed up past, she believes the only person who ever loved her is her adoptive grandfather. He comes up with the belief of the Hinamizawa Syndrome, which is where Hinamizawians have mass histeria if they are to leave Hinamizawa because of a disease. Nobody 'important' believes him, which makes Takano very angry, because she helped her grandfather. After her grandfather dies, she adopts the study of the Hinamizawa Syndrome, and brings it up with a group called 'Tokyo'. They allow her to go to Hinamizawa with a two of their employees, Irie and Tomitake. While there, they try to 'fit in', so they develope aliases.Very ironically, being around the Hinamizawa Syndrome the most made Takano catch it, making her crazy to prove her grandfather right. Ultimately, she does, because she comes down with it and proves it's symptoms by doing the symptoms, which is to kill almost uncontrollably. She kills everyone because she has the Hinamizawa Syndrome herself.