you have to remember though that fukawa is not the same person as genocider syo. yes, they share the same body, but they are entirely different people. fukawa cannot control what syo does. she absolutely hates syo, and when chihiro dies, she freaks out and hides away because she literally does not want to become syo again. also, she's afraid that syo killed chihiro because if that was the case, she was going to get executed.
and the reason fukawa has trust/social issues is because everyone has always been like that to her. all of the boys she's ever gone on a date with did it as a dare, or to mock her. not to mention she's quite possibly from an abusive household, and there's even evidence that her mother has DID.
idk fukawa is just a very interesting character.