I think you might be misremembering. When the detective asked Sei, she put her arm on his shoulder and was looking at him, and with the way it was framed it seemed entirely calculated.She seemed to have accepted her fate when the detective asked her son
@softhaneul thank you for putting two warnings i have a bad habit of pressing spoilers the moment i see thembig spoileraccidentally read that sei was the one that pushed shigeru
so this make sense