That was hell of a dangerous thing to do while Saitou was on her herbal medicine, they were supposed to make happy memories, but made a deep cut that was necessary for her to stop being unnecessarily jealous of anyone and hurting them without knowing their true intentions.
This was really dark at the end, Yokoi should know by now that if Saitou really snaps, she won't try cutting her wrist but go to the forest where nobody will be able to help.
Ryuunosuke really is a piece of shit, future landlord of the year.