Conan solves the case and Yumekawa admits to committing the crime, she basically thought of the trick because she originally planned to sell it as material to a famous writer, but she didn't have confidence that the trick would actually work, so she talked about it to the three victims online.
So the three tested it at home, and it was a success however before she could sell it, Morita (one of the victims) used it in the online novel she was writing as a hobby, and uploaded it.
she got angry and asked her to delete it at once, sent her flying, then apologized but ended up killing her, then she used the trick.
However the two others might also realize the trick, so she both called them one after the other saying "the culprit ain't me nor you, so it has to be the last one, let's meet and ask for help" and killed them too.
She could've refused the interview by saying "i can't get back from my best friends' deaths" but she accepted because she couldn't sleep thinking it might come out to the light someday, so she thought that if she was able to deceive Yusaku, she'd be relieved afterwards and could sleep.
Megure mentions that Kid was sighted in Shibuya at the same time and Conan realizes that the Yusaku is not Kid. Turns out Kid was Vermouth following Rum orders. The BO is preparing a big move.
