The culprit is Tadahiko Michiwaki He kills brown-haired girls out of resentment toward the brown-haired girl who broke up with him. However he target Sonoko for another reason. When he was killing he latest victim a train passed by behind him and he felt light from the air cables. He mistook it for a flash from a camera and when he soon after saw Sonoko pass by the trails with a camera, he assumed that she took a picture of him with the victim, so he approached the group with the intention of destroying that picture. He even feared that Sonoko was going to show it to Shinichi, when she talked about the picture of Ran. And since all of Sonoko's pictures were of couples, he thought that she might have taken the picture in the belief that it was another couple in an intimate moment.
