[Klashikari]

probably because of the local religion.

remember about how the dam war occured : the furious people were acting panzer all the way.

in fact, the watanagashi style gives 2 undeniable advantages :
1) the police, even while carrying every possibilities, will be focusing on a mere "random culprit" speculation which is simply a fanatic.
2) the chaos which rika's death under "oyashiro-sama curse" will probably generate some crazed people, which will not only give to takano perfect scapegoat for her cover

but also, takano's disaster plan require a good excuse to eliminate hinamizawa people. and what is it? the hinamizawa syndrome, which cannot be controled anymore (the queen died), and will let loose the paranoid frenzied people. (a huge treat to the nation, which led fear amoung the country people and possibility lots of chaos around the area)

for that, she need to put the people under the paranoid killing frenzy with some measure and countermeasure (fear from oyashiro with rika watanagashi death, some artificial madness like for tomitake's ).

with this, she proves her grandfather theory, but also get the authority to suppress hinamizawa village.

in fact, the 2 factors are merged : 1) the proof of the theory 2) the excuse to kill everyone